Elisabetta Cocciaretto and Jasmine Paolini both won their singles matches to help their country triumph in Shenzhen and prolonged the American wait for another title after their previous success in 2017.
World number eight Paolini's 6-4, 6-2 victory was her maiden win in her sixth clash with Jessica Pegula and helped Italy become the first team to capture back-to-back titles since the Czech Republic lifted three straight trophies from 2014-16.
"It's been a really great week, and it's always amazing to play in this competition... I just want to thank everybody, it was unbelievable," a beaming Paolini said.
"It was so tough, this year we played amazing teams, they are very strong and today against the United States, we didn't expect to win, so we're really happy about this win.
"I don't know, it's even more special this year than last year. It's different, but honestly, I didn't expect it."
The 29-year-old edged a tight first set by firing a backhand rocket that broke the seventh-ranked Pegula's serve, and she came from an early break down in the next set to quell the challenge of her American opponent after a late wobble.
She celebrated her win by raising her arms and joined her team in dancing in the centre of the blue court to roaring applause from largely Italian and Chinese fans.
Earlier, Cocciaretto ensured that Italy shot out of the blocks by swatting aside world number 18 Emma Navarro 6-4, 6-4, with the 24-year-old finishing off in style by winning a breathtaking rally on match point.
"It was an unbelievable match for me," said world number 91 Cocciaretto, who battled from 2-4 down in the second set to get past her opponent in 89 minutes at a packed Shenzhen Bay Sports Centre Arena.
"I knew she's a really great player and I had to play my best tennis ... I'm really happy about the performance and the point I gave to Italy."
Italy's sixth Billie Jean King Cup title comes amid a golden spell for the country, with the men's team claiming the previous two Davis Cup trophies and their players enjoying success at the Grand Slam and Olympic level.