Concerned by the low numbers of law students choosing careers in public service, Harvard Law School plans to waive tuition for third-year students who pledge to spend five years working either for nonprofit organizations or the government.

The program, to be announced Tuesday, would save students more than $40,000 in tuition and follows by scant months the announcement of a sharp increase in financial aid to Harvard’s undergraduates. The law school, which already has a loan forgiveness program for students choosing public service, said it knew of no other law school offering such a tuition incentive.