If the President can no longer serve, who becomes President?

If the President of the United States is unable to serve, the Vice President becomes President. This is clearly laid out in the 25th Amendment to the United States Constitution.

The amendment was ratified in 1967, partly in response to the assassination of President John F. Kennedy in 1963, and the subsequent concerns about the continuity of government functions. It specifically provides for the succession and replacement of the President and Vice President, including clarification that the Vice President becomes President (rather than just acting as President) if the President dies, resigns, or is removed from office.