The victory, coupled with the Angels' 3-1 home loss to the Oakland Athletics Friday, secured the Rangers back-to-back division crowns for the first time since 1998-99.
"It feels great simply because it's so hard to repeat," said Texas manager Ron Washington.
Matt Harrison (14-9) won his fourth straight start after scattering three hits over six solid innings. The left-hander gave up all three runs in his final frame of work.
Neftali Feliz pitched around a two-out walk in the ninth to earn his 30th save of the season.
Seattle rookie Anthony Vasquez (1-5), a Texas native, surrendered the home runs by Beltre and Gentry as well as a solo blast from Josh Hamilton among seven hits in 3 1/3 innings.
Texas got on the board in the second when Beltre followed Michael Young's leadoff single with a smash to left field. Beltre's 29th home run upped his RBI total this year to 100, the second straight season he reached triple digits.
The Rangers loaded the bases later in the inning, but Vasquez escaped further damage with a double play.
Gentry recorded an inside-the-park home run after Hamilton went deep in the third. Nelson Cruz led off the fourth with a single and Gentry lofted a fly ball that split left fielder Mike Carp and center fielder Trayvon Robinson, who both dove on the play, and rolled on the way to the wall. Gentry raced around the bases for his first major league home run and a 5-0 Texas lead.
"When you're playing teams down the stretch that are in it, our guys are going to get better each and everyday just from having those experiences," said Seattle manager Eric Wedge.
The last Ranger to hit an inside-the-park home run was Cruz on September 4, 2006 at Oakland...Texas is 13-4 against Seattle this season.