The Lady Tigers (23-9), seeded second, ended the 12-game winning streak of top-seeded and SWAC co-champion Grambling (18-14) by scoring the last seven points of the game.
Breasia McElrath added 10 points for Texas Southern, which played in three-straight WNIT Tournaments from 2013-2015. Prior to that, Texas Southern made back-to-back appearances in the NAIA Tournament in 1981 and 1982. In 2014, Texas Southern finished as the runner-up in the SWAC Tournament, the closest the program had ever gotten to an NCAA berth in recent years.
Grambling State was led by John'ea Thompson and Shakyla Hill with 23 and 21 points, respectively.
Grambling State led most of the first half, the big lead being eight, as Thompson scored 16 points.
Trailing 30-22 with 4:40 left in the second quarter, Texas Southern rattled off a 10-0 run in a span of 1:26, leading to a 35-35 halftime tie.
In the third quarter, Grambling State and Texas Southern traded run for run, with neither team being able to pull away. By game's end there were 14 lead changes and seven ties.
With just 10 seconds left at the end of the third, Grambling State's Bre'Nae Andrews scored her only a basket, a 3, for a 51-49 lead entering the fourth quarter. Grambling State had missed its first 10 3s and finished 3 of 17.
In the opening 4:24 of the fourth quarter, Texas Southern outscored Grambling State 11-5 to stretch its lead out to 60-56.
Trailing 66-63 with 2:29 left in the game, Texas Southern's Chynna Ewing came up big netting back-to-back buckets for a 67-66 lead.
With eight seconds left, after a steal by Hill, Grambling State's Chantiara Lewis missed a layup, which would have tied the game.
Texas Southern sealed the victory at the free throw line hitting 3 of 6 at the charity stripe in the final seconds.
Grambling, which hadn't won a league title in 17 years, hasn't been to the NCAA Tournament since 1999.
The Associated Press contributed to this report.
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