SAN FRANCISCO, 11 July 2016, (AFP) - Nordqvist was penalized for grounding her club in a fairway bunker, with video showing her club lightly touching the sand before she hit out.
That paved the way for Lang -- who parred all three playoff holes -- to capture her first major title at CordeValle in San Martin, California.
The two had ended regulation tied on six-under par 282 -- two shots in front of a group that included world number one Lydia Ko.
Nordqvist, who had stormed to the top of the leaderboard with a final-round 67 that included three birdies and an eagle, didn't learn of the penalty until after her third shot at the final playoff hole.
"Obviously that was very disappointing," Nordqvist said. "I didn't do it on purpose. What can I say, I touched the sand. It's hard to lose that way."
America's Lang, who captured her only prior LPGA Tour title at the 2012 Manulife Financial LPGA Classic, closed with a one-under 71.
She had taken sole possession of the lead with a birdie at the par-three 16th, but was tied for the lead with Nordqvist after a bogey at 17.
Ko, seeking at 19 to become the youngest golfer to win a third major title, led after three rounds and had stretched her advantage to two strokes with a birdie at the sixth.
But the New Zealand followed a bogey at the eighth with a double-bogey at the ninth.
"When you're doubling a par-five, it doesn't help the scorecard," acknowledged Ko, who had two more bogeys and a birdie coming in.
South Korea's Park Sung-Hyun, who led after 36 holes, bogeyed the last to complete a 74 that left her tied for third on 284 alongside Ko and South Koreans Amy Yang (73) and Ji Eun-Hee (74).
Leading final-round scores on Sunday in the US Women's Open at CordeValle in San Martin, California (x-won in three-hole aggregate playoff; par-72, USA unless stated):
282 - x-Brittany Lang 68-75-68-71, Anna Nordqvist (SWE) 68-74-73-67
284 - Amy Yang (KOR) 67-71-73-73, Ji Eun-Hee (KOR) 69-71-70-74, Lydia Ko (NZL) 73-66-70-75, Park Sung-Hyun (KOR) 70-66-74-74
285 - Stacy Lewis 71-74-69-71
286 - Jodi Ewart Shadoff (ENG) 70-71-75-70, Gerina Piller 70-72-74-70, Cristie Kerr 67-75-72-72
287 - Ryu So-Yeon (KOR) 71-76-71-69, Sydnee Michaels 69-72-75-71, Lee Mi-Rim (KOR) 64-74-76-73, Haru Nomura (JPN) 70-69-75-73, Gaby Lopez (MEX) 71-72-71-73, Angela Stanford 71-70-71-75
288 - Lee Lopez 73-72-71-72, Jessica Korda 70-70-75-73, Ariya Jutanugarn (THA) 70-75-69-74, Danielle Kang 71-69-73-75
289 - Jang Ha-Na (KOR) 73-71-74-71, Suzann Pettersen (NOR) 72-74-72-71, Mo Martin 71-70-77-71, Alena Sharp (CAN) 70-72-75-72, Kris Tamulis 71-72-71-75
290 - Christina Kim 70-75-73-72, Maude Leblanc (CAN) 72-69-76-73, Lin Xiyu (CHN) 73-73-73-71, Lizette Salas 70-78-71-71, Catriona Matthew (SCO) 73-72-71-74, Kim Sei-Young (KOR) 75-68-72-75