The MP for Leeds West will be shadow chancellor of the Duchy of Lancaster.
On Twitter, she said "it is a huge honour to be appointed", adding: "I look forward to working with Keir and Team @UKLabour as we provide a constructive opposition at this incredibly difficult time for our country."
Ms Reeves was previously Shadow Chief Secretary to the Treasury between 2011 and 2013, and Shadow Secretary of State for Work and Pensions between 2013 and 2015.
Former treasury minister Anneliese Dodds has been made shadow chancellor, Sir Keir's leadership rival Lisa Nandy has been appointed shadow foreign secretary and Nick Thomas-Symonds will be shadow home secretary.
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Nick Brown has been re-appointed chief whip and Angela Smith remains shadow leader of the Lords.
Jonathan Ashworth will remain shadow health secretary.
John McDonnell and Diane Abbott, who served under Jeremy Corbyn, are out of the shadow cabinet, but former shadow foreign secretary Emily Thornberry is expected to remain - albeit in a different role.
Sir Keir said: "We are living through a national emergency. Under my leadership, the Labour Party will always act in the country's interest to save lives and protect livelihoods. That will be the number one priority of my shadow cabinet.
"We will be a responsible opposition that supports the Government where we believe they are right and challenge them when we believe mistakes are being made."
Barry Gardiner, Jon Trickett and Ian Lavery have left the shadow cabinet, while appointments are set to be finalised on Monday.
Sir Keir pledged earlier on Sunday to create a "balanced" team from across the country and party, with people who "want to serve towards the future aim of winning that next general election".
He has vowed to make it his "mission" to reconnect the party with the public, saying Labour needs to change so trust can be regained.
Sir Keir secured 56 per cent of the 490,731 votes cast in the three-month contest - beating his rivals Rebecca Long-Bailey and Lisa Nandy to replace Jeremy Corbyn.
"I will have in my shadow cabinet those that want to serve towards the future aim of winning that next general election," he told BBC One's The Andrew Marr Show.
"It will be a talented, balanced shadow cabinet."
He has vowed to engage constructively with the Government amid the coronavirus crisis, saying he will not seek to score party political points - nor demand the impossible.
On Brexit, Sir Keir said the Government "should extend" the transition period "if it's necessary to do so" because of the pandemic.
Angela Rayner won the deputy leadership with 52.6 per cent of the vote in the third round, and promised to "do everything" to repay her supporters' trust.
She acknowledged the party had "let down" the Jewish community and also has to win back the respect of voters who had left the party to vote Tory.