His last boxing bout, a two-round TKO loss to Cecil McKenzie in a California State Cruiserweight title fight, took place on July 10, 2003.
On February 26, 2006 Nishijima debuted in mixed martial arts, at PRIDE 31, taking on Mark Hunt and losing by KO.
Currently Nishijima's record in mixed martial arts is 0–5 with most recent loss to Melvin Manhoef by TKO, took place on December 31, 2007 at K-1 PREMIUM 2007 Dynamite!!.
He lost to Peter Aerts by KO due to a right leg kick on August 11, 2009 at the K-1 World Grand Prix 2009 in Tokyo Final 16 Qualifying GP.
In his retirement match, Nishijima defeated Bob Sapp under kickboxing rules at Legend 4 in Kumamoto, Japan on November 17, 2013, although he was not allowed to kick as he chose to wear boxing shoes.