Dr Wollaston, who announced the move late on Wednesday, quit the Conservatives in February over the party's "disastrous handling of Brexit".
The Totnes MP joined what would become Change UK, but left that fledgling group in June after its dismal performance in the European Parliament elections.
She now becomes the latest politician to join the resurgent Remain-supporting Lib Dems, following Chuka Umunna's similar pathway from Labour.
Dr Wollaston said: "I believe the best way for me to represent my constituents in Totnes is to be working as part of a fantastic team of Liberal Democrat MPs who are unequivocally making the case for us to remain at the heart of Europe, as well as campaigning for social justice, the environment and our public services."
Lib Dem leader Jo Swinson said she was "delighted" to welcome Dr Wollaston.
"We have worked together for many months to fight to stop Brexit and, as one of the most respected Members of Parliament, she brings real expertise to our team," Ms Swinson added.
"As the strongest party for Remainers, we will lead the fight to stop Brexit."