Shelby Rogers and Coco Vandeweghe downed Aryna Sabalenka and Aliaksandra Sasnovich 6-3 7-6(3) in the doubles to hand the visitors the decisive point.
The U.S. had not won the title since beating Spain 5-0 in 2000, while Belarus were without former world number one Victoria Azarenka, who missed the final due to an ongoing custody battle with her baby son's father.
Vandeweghe is the first player to record a spotless year in Fed Cup singles rubbers since Petra Kvitova went unbeaten for the Czech Republic in 2011, and the American won eight matches in total - six singles and two doubles - in the competition.
Kathy Rinaldi was understandably over the moon after becoming the first U.S. captain to guide a team to a Fed Cup title in their first year since Marty Riessen in 1986.
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"I believe in these American players," she said in a courtside interview. "They're tough as nails. American women are tough as nails! There's going to be dancing all night."
Vandeweghe had put the visitors ahead with a composed display to beat world number 78 Sabalenka 7-6(5) 6-1, before Sasnovich struck back, downing Sloane Stephens 4-6 6-1 8-6 to force the doubles.
Rinaldi and Belarus captain Eduard Dubrou sprung surprises for the decider, with the former sending out Vandeweghe instead of Alison Riske, and Dubrou sticking with Sabalenka and Sasnovich instead of fielding Vera Lapko and Lidziya Marozava.
"I'm just really thrilled for all four players," Rinaldi added. "I'm so proud of all four of them. That was the plan, to go out there and take the doubles.
"Coco came out and played just unbelievable. Shelby stepped up and Sloane, my heart was broken for her but she battled this whole week and she battled all the way to the end."
U.S. Open champion Stephens lost both her matches and her defeat on Sunday means she has not won a singles tie since winning the final at Flushing Meadows on Sept. 9.
(Writing by Simon Jennings in Bengaluru, editing by Pritha Sarkar and Christian Radnedge)
