Pitts made 7 of 12 shots, including 4 of 6 outside the arc, for sixth-seeded Marshall (21-12), which will play Saturday in its first conference tournament final since the 1997 Southern Conference championship game. The Thundering Herd, which beat Tulsa 105-100 in three overtimes Thursday, will face top-seeded Memphis, an 83-52 winner over Central Florida.
DeAndre Kane added 14 points for Marshall, and Dennis Tinnon had 10 points and 13 rebounds.
Southern Mississippi (25-8), the second seed, was led by Angelo Johnson's 24 points. Darnell Dodson added 13.
Marshall shot 54.3 percent (25 of 46), including 9 of 17 (52.9 percent) on 3-pointers. That offset 16 turnovers by the Thundering Herd, compared to three by the Golden Eagles, who shot 34.2 percent (25 of 73).
"I thought our defense was outstanding," Marshall coach Tom Herrion said. "We were so locked in defensively. . I thought we guarded really, really well."
The two teams were close through much of the game until Marshall used a 17-4 rally to turn a 52-47 Southern Mississippi lead with 10:05 remaining into a 64-56 advantage for the Thundering Herd. Pitts capped the run with eight straight Thundering Herd points.
"They got out on us for a little bit and got a few easy buckets in transition," Pitts said of Southern Mississippi taking the lead. "We just had to settle down on defense and get a few stops and a couple of easy baskets."
The game was closely played in the first half with Marshall carrying a 29-28 lead into the break. Kane, who was coming off a 40-point performance in the triple-overtime win over Tulsa, led Marshall with 10 points at halftime.
No Golden Eagles were in double figures for the first half as Southern Mississippi shot 36.4 percent (12 of 33). Part of that came from the inside defensive presence of Marshall's Nigel Spikes, who had a couple of blocks.
Despite its shooting woes, Southern Mississippi was able to stay close by protecting the ball, committing only one turnover in the half compared to nine by Marshall, which held the largest lead of the half at six points.
"We kind of got what we wanted on offense today," Herrion said. "If we didn't puke it up 16 times, Lord knows. They had 20 points off our turnovers. That was a third of their points."
The game remained tight in the second half. There were seven ties in the first 12 minutes, as neither team could gain much of a buffer. Southern Mississippi got the lead to 52-47, but Marshall then ran off nine straight points.
"I thought the tougher team won today," Southern Mississippi coach Larry Eustachy said. "... You saw those two teams battle it out. We've got a five-point lead and we had some lapses defensively. Our offense dictated our defense a little bit, which isn't our game."