His withdrawal leaves just three candidates battling it out to be the next prime minister- May, justice secretary Michael Gove and energy minister Andrea Leadsom.
Speaking to the BBC, Crabb said he would offer his "wholehearted support" to the home secretary, and described her as "the only one candidate in a position to unite our party and lead a strong and cohesive government".
Tory backbencher Liam Fox was eliminated earlier tonight in the first round of voting among Tory MPs. Since then, the former defence secretary has also backed May.
Crabb came fourth in the poll, securing the backing of just 34 MPs.
By contrast, May secured a resounding victory with 165 parliamentarians declaring their support, just over half of the total voting.
Leadsom was backed by 66 MPs, while Gove secured 48 votes.
THE FULL RESULTS
- Theresa May - 165
- Andrea Leadsom - 66
- Michael Gove - 48
- Stephen Crabb 34
- Liam Fox - 16
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