North Side advances to take on Ben Davis (22-5) at Bankers Life Fieldhouse in Indianapolis next Saturday night.
A Juan Quarles bucket with 6:19 left in regulation put the Legends up 64-43, but the Pirates went to work. Quarles, who finished with 16 points and six rebounds, fouled out shortly thereafter and, with Keion Brooks playing with four fouls, Merrillville got aggressive. The lead would shrink to as little as six at 79-73 late in the fourth quarter.
North Side shot just 22-of-39 from the free throw line, but hit 8-of-10 over the final 1:15 to survive.
Brooks finished with a team-high 20 points for thr Legends, while Davontae Kinnie and Brandan Johnson added 15 each. Jaylen Butz went for 13 points and 18 rebounds.
Merrillville (21-8) was paced by sixth man Johnny Bernard, who dropped 32. Cameron Wilbon added 16.
North Side last appeared in the state championship in 1965 when the then-Redskins under Coach By Hey lost to Indianapolis Washington 64-57.
The Class 4A championship game is tentatively scheduled to tip off at approximately 8:15 p.m. next Saturday night.