President Donald Trump had repeatedly blasted Sessions over his decision to recuse himself from an investigation into Trump's electoral campaign. How Sessions' exit will affect that probe is still unclear.
Sessions gave Trump an undated resignation letter.
The letter says the resignation was at Trump's "request."
Trump announced Sessions' departure on Twitter and wrote: "We wish him well."
Whitaker will be acting attorney general until Trump names a permanent replacement.
Whitaker has previously criticized the Mueller probe, once musing about whether an attorney general could undermine the investigation by cutting its budget.
What did Sessions write in his resignation letter?
In his undated letter, Sessions wrote he was "submitting his resignation" at the request of the president.
Sessions said he had advanced Trump's political objectives during his time as attorney general. He listed spearheading a crackdown on irregular migrants, breaking up violent "transnational gangs" and tackling the opioid epidemic among his achievements.
"Most importantly, in my time as attorney general, we have restored and upheld the rule of law — a glorious tradition that each of us has a responsibility to safeguard."
Democrats warn Trump
Nancy Pelosi, the Democrat expected to lead the party in the House of Representatives, said the resignation was a "blatant attempt" by Trump to undermine special counsel Robert Mueller's investigation into possible Russian collusion with the Trump campaign.
"Given his record of threats to undermine & weaken the Russia investigation, Matthew Whitaker should recuse himself from any involvement in Mueller’s investigation," wrote Pelosi on Twitter.
The Democrats' leader in the Senate, Chuck Schumer, said both houses of the US Congress needed to protect the probe.
"It would create a constitutional crisis if this were a prelude to ending or greatly limiting the Mueller investigation," he said.
Dianne Feinstein, the top Democrat on Senate Judiciary Committee, also said Whitaker should "make a firm commitment not to interfere in the investigation."
Who is Jeff Sessions?
The Alabama Republican was the first senator to back Trump in the 2016 presidential election. Trump started publicly criticizing him after Sessions recused himself from the Russia investigation in early 2017.
A timeline of the Russia investigation
Allegations of collusion with the Kremlin have dogged Team Trump since the 2016 election campaign. DW takes a look at how special counsel Robert Mueller's Russia investigation unfolded.
Image: Reuters/L. Downing
2013: Mr. Trump goes to Russia
June 18, 2013. Donald Trump tweeted: "The Miss Universe Pageant will be broadcast live from MOSCOW, RUSSIA on November 9. A big deal that will bring our countries together!" He later added: "Do you think Putin will be going - if so, will he become my new best friend?" October 17, 2013 Trump tells chat show host David Letterman he has conducted "a lot of business with the Russians."
Image: picture-alliance/dpa/V. Prokofyev
September 2015: Hacking allegations raised
An FBI agent tells a tech-support contractor at the Democratic National Committee it may have been hacked. On May 18, 2016, James Clapper, the director of National Intelligence, says there were "some indications" of cyberattacks aimed at the presidential campaigns. On June 14, 2016 the DNC announces it had been the victim of an attack by Russian hackers.
Image: picture alliance/MAXPPP/R. Brunel
July 20, 2016: Kislyak enters the picture
Senator Jeff Sessions — an early Trump endorser who led his national security advisory committee — meets Russian Ambassador Sergey Kislyak and a group of other ambassadors at a Republican National Convention event.
Image: Getty Images/AFP/B. Smialowski
July 22, 2016: Assange thickens the plot
Julian Assange's WikiLeaks publishes 20,000 emails stolen from the DNC, appearing to show a preference for Hillary Clinton over Senator Bernie Sanders.
Image: Reuters/N. Hall
July 25, 2016: Cometh the hour, Comey the man
The FBI announces it is investigating the DNC hack saying "a compromise of this nature is something we take very seriously."
Image: Getty Images/AFP/B. Smialowski
November 8, 2016: Trump elected
Donald Trump is elected president of the United States. On November 9, the Russian parliament burst into applause at the news.
Image: Reuters/K. Lamarque
November 10, 2016: Team Trump denies Russia link
Russian Deputy Foreign Minister Sergey Rybakov says there "were contacts" between the Russian government and the Trump campaign during the election campaign. The Trump campaign issues a firm denial.
Image: Imago/Itar-Tass
November 18, 2016: Flynn appointed
Trump names General Michael Flynn as his national security adviser. The former Defense Intelligence Agency chief was a top foreign policy adviser in Trump's campaign. Flynn resigned in February after failing to disclose full details of his communication with Russian Ambassador Sergey Kislyak.
Image: Reuters/C. Barria
January 26, 2017: Yates - 'The center cannot hold'
Acting Attorney General Sally Yates tells White House counsel Don McGahn that Flynn made false statements regarding his calls with Kislyak. On January 30, Trump fires Yates for refusing to enforce his travel ban, which was later blocked by federal courts.
Image: Getty Images/P. Marovich
March 2, 2017: Sessions recuses himself
Trump says he has "total confidence" in Attorney General Jeff Sessions. Sessions announces he will recuse himself from any investigation into ties between Russia and the Trump campaign.
Image: Getty Images/S.Loeb
March 20, 2017: FBI examines Trump-Kremlin links
FBI Director James Comey confirms before the House Select Committee on Intelligence that the FBI was investigating possible links between Russia and the Trump campaign.
Image: picture-alliance/dpa/AP/J. S. Applewhite
May 9, 2017: Trump sacks Comey
In a letter announcing the termination, Trump writes: "While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the judgment of the Department of Justice that you are not able to effectively lead the Bureau."
Image: Reuters/J. Ernst/K. Lamarque
May 17, 2017: Mueller appointed special counsel
Deputy Attorney General Rod Rosenstein appoints former FBI Director Robert Mueller to look into Russia's interference in the 2016 election and possible collusion with the Trump campaign.
Image: picture-alliance/AP Photo/J.S. Applewhite
August 2017: FBI seizes documents from Manafort
Shortly after Mueller convenes a grand jury for the investigation, the FBI seizes documents from one of Paul Manafort's properties as part of a raid for Mueller's probe. The former Trump campaigner manager stepped down in August 2016 after allegations surfaced that he had received large payments linked to Ukraine's former pro-Russian government.
Image: Imago
September 2017: Trump Jr.'s talks to Senate committee
Donald Trump Jr. tells the Senate Judiciary Committee he has not colluded with a foreign government. The closed-door interview relates to his June 2016 meeting with Russian lawyer Natalia Veselnitskaya, which was also attended by his brother-in-law, Jared Kushner, and then-campaign manager Paul Manafort. Trump Jr.’s emails, however, suggest the meeting was supposed to produce dirt on Clinton.
Image: picture alliance/AP Photo/K. Willens
October 2017: Internet giants allege Russian interference
Facebook, Twitter and Google reportedly tell US media they have evidence that Russian operatives exploited platforms to spread disinformation during the 2016 US presidential election. The three companies are appear before the Senate Intelligence Committee in November 2017.
Image: picture-alliance/dpa/Lei
July 2018: Trump and Putin meet in Helsinki
Trump and Russian President Vladimir Putin meet in Helsinki for their first-ever summit. During the trip, Trump publically contradicts the findings of US intelligence agencies who concluded that Russia interfered in the 2016 presidential election.
November 8, 2018: Sessions resigns as attorney general
Attorney General Jeff Sessions resigns from his post, under reported pressure from Trump. The president then appoints a critic of the Mueller probe as his successor, but later nominates William Barr to be the next attorney general in December 2018.
Image: picture-alliance/AP/A. Brandon
November 29, 2018: Former Trump lawyer pleads guilty
Trump's former long-time personal lawyer, Michael Cohen, pleads guilty to lying to Congress about discussions in 2016 on plans to build a Trump Tower in Moscow. The FBI raided his home earlier that year in April. He would later be sentenced to three years in prison. In 2019, he tells Congress that Trump is a "racist" and a "con man."
Image: Reuters/J. Ernst
January 2019: Trump associate Roger Stone arrested
Roger Stone, a longtime Trump associate and Republican operative, is arrested at his home in Florida for lying to Congress about having advance knowledge of plans by WikiLeaks to release emails from the Democratic Party that US officials say were stolen by Russia.
Image: picture-alliance/dpa/AP Photo/L. Sladky
March 13, 2019: Manafort sentenced to prison
Manafort is found guilty of conspiracy charges and handed an additional sentence, bringing his total prison sentence to 7.5 years. In August 2018, a court in Virginia found him guilty of eight charges, including tax and bank fraud. He also pleaded guilty to two conspiracy counts.
Image: picture-alliance/AP/D. Verkouteren
March 22, 2019: Mueller ends Russia probe
Special counsel Robert Mueller submits a confidential 448-page report on the findings of his investigation to the US Justice Department. The main conclusions of the report are made public when they are given to Congress. A redacted version of the report is released to the public on April 18, though Democrats call for the full report to be released.
Image: picture-alliance/AP/C. Dharapak
March 24, 2019: Trump declares 'exoneration'
The final report concluded that no one involved in Trump's 2016 election campaign colluded with Russia. Attorney General William Barr said the report provided no evidence that Trump obstructed justice, but stopped short of fully exonerating the president. Reacting to the findings, Trump described the probe as an "illegal take-down that failed," and said there was "complete and total exoneration."
Image: Reuters/K. Lamarque
May 1, 2019: Barr testifies
In late March, Mueller writes a letter expressing concerns over the way Barr portrayed his report. The attorney general says the special counsel's letter was "a bit snitty" while testifying in front of the Senate Judiciary Committee in May. Barr then cancels a subsequent appearance before the House Judicial Committee, citing "unprecedented and unnecessary" hearing conditions.
Image: Getty Images/A. Wong
July 24, 2019: Mueller light
Robert Mueller's congressional testimony on the Russia probe was again inconclusive. He sometimes struggled with his answers or avoided queries. To the Democrats frustration he appeared to do little to give any encouragement to the notion that President Trump could be impeached, though he did suggest he might be prosecuted for obstruction of justice crimes after he leaves the White House.
