Jim Belushi -- Tupac Was a HUGE Frank Sinatra Fan

Gangster rap icon Tupac Shakur secretly LOVED sappy love songs ... especially Frank Sinatra's ... so says Jim Belushi, who's now taking credit for inspiring Tupac's soft side.

Jim was out in L.A. Tuesday when we asked if Tupac inspired him while they worked on "Gang Related" together -- Tupac's final film role before the rapper's death in 1996.

Jim hesitated ... then said, "I influenced him. I turned him on to Frank Sinatra."

Jim said he and Pac listened to a certain Sinatra song together and tried to figure out how to remix it into a rap song ... but Pac said, "This [song] is so melodic. This song is so beautiful, it should never be touched."