The judge awarded life imprisonment to three others - Shamsher Singh, Gurmeet Singh and Lakhwinder Singh - who were convicted last week for conspiring to assassinate Beant Singh.
A suicide bomber, Dilawar Singh, killed the chief minister as he came out of his office at the high security Punjab civil secretariat here Aug 31, 1995.
Balwant Singh was the second human bomb, to be used in case the first suicide bomber failed to kill Beant Singh.
A sixth person convicted in the same case under the Explosives Act, Naseeb Singh, was awarded 10 years imprisonment by the court on Tuesday. Since he has already spent 12 years in prison, he is likely to be let off.
All six were last week convicted by the special Central Bureau of Investigation (CBI) court in the conspiracy of assassinating Beant Singh, who became chief minister in 1992.
Beant Singh, along with "super cop" KPS Gill, was largely credited with wiping out terrorism from Punjab by dealing with terrorists with an iron hand.