Writing to Twitter on Saturday, Sir Keir said he felt "delighted" that Ms Berger had accepted his invitation to rejoin her former party, stating: "My test for change was whether those who were rightly appalled by how far we had fallen believe this is their party again.
In letters between the pair shared by Sir Keir, the Labour leader apologised that his former colleague had been "forced out by intimidation, thuggery and racism" and said she was "principled and brave".
Ms Berger said she believed there had been a "grim journey" between 2015 and 2019 "during which the party fell into the depths of the abyss under Jeremy Corbyn's reign".
She added that she was looking forward to rejoining Labour and "working with" Sir Keir to "continue what you have started".
"It is time to replace this reckless and divisive government and ultimately make the difference our country so desperately deserves,” she continued.
Sir Keir responded that Labour and British politics as a whole were "poorer places" without her."The abuse you suffered was disgusting. You were left isolated and exposed. Shamefully, those who should have defended you stood by,'' he continued.
"The Labour Party - our party - has always prided itself on being a party of equality, collectivism, solidarity, and anti-racism.
"But during those dark days, we were none of those things."
In 2020 a report by human rights watchdog the Equality and Human Rights Commission concluded that the party was responsible for unlawful instances of harassment and discrimination.
Former party leader Mr Corbyn rejected elements of the report and said allegations of antisemitism under his leadership had been "dramatically overstated for political reasons".
He was subsequently suspended from the Labour party.
Earlier this month Sir Keir said Mr Corbyn would not be permitted to stand as a Labour candidate in the next general election scheduled for 2024.
After quitting Labour in 2019 Ms Berger initially sat with seven other Labour MPs who had also left the party as independents forming the anti-Brexit Change UK party.
She later left the faltering new party to sit as an independent before joining the Liberal Democrats in September 2019.
At the December 2019 general election, she stood unsuccessfully for Finchley and Golders Green. She is currently serving as CEO of a Westminster-based public affairs firm.
