The 27-year-old scored 23 goals this season but is now looking for a new club.
Goalkeeper Dean Brill, defenders Sam Cox, Darren Dennehy and Jordan Parkes, midfielder Alassane N'Diaye and winger Mark Marshall - who is serving a two-year drugs ban - are also departing Underhill.
The Bees are still in talks with skipper Mark Hughes, fellow midfielder Sam Deering and defender Jack Saville about their futures.
Barnet are continuing their search for a new manager after Martin Allen's successful short-term stint, where they escaped relegation on the final day of the season, came to an end.
Director of football Paul Fairclough is leading the hunt and, while Allen has admitted he would like the job permanently, the club say they have "not ruled anyone out of the search".
