Lawrence, 49, who is best known for her appearances on Whose Line Is It Anyway?, will play Manda Best, Phil's childhood sweetheart.
Best returns to Albert Square following the collapse of her marriage and her son's departure for university. Her character quickly begins a relationship with Minty (Cliff Parisi), but it is clear that she still has affections for Phil (Steve McFadden).
An Albert Square source told DS: "Manda returns to her roots in Walford for a specific storyline with Minty and Phil."
They added: "Beyond that, though, there are plenty of potential storyline opportunities for her to able to stick around."
Lawrence told The Sun: "I'm so excited. Everyone at EastEnders has made me feel so at home. It's mind-boggling to walk into Albert Square after having watched the show for so many years! I can't wait to see how my character develops on screen."
The soap's executive producer Diederick Santer commented: "I'm so excited Josie’s joining us. I've enjoyed her brilliant comic performances for over 20 years, and can't wait to see her in Walford."
Lawrence's first scenes will air in March.
At the beginning of the week, it was announced that Rolan Bell would also be joining the Walford-based soap as mystery character Theo Kelly.