Long-striding Hudson-Smith seemed on course to win his country’s first gold over the distance since "Chariots of Fire" Eric Liddell in the Paris Games 100 years ago, but Hall swept past to win in a personal best 43.40 seconds and give the U.S. their first triumph since LaShawn Merritt in Beijing.
Hudson-Smith bettered his own European record with 43.44 for silver and Zambia's 21-year-old Muzala Samukonga set his second successive national record in Paris with 43.74 to take bronze.
Hall, who took bronze behind Jamaica’s Antonio Watson and Hudson-Smith at last year’s world championship, looked out of contention coming into the final straight, a distant fourth as Hudson-Smith seemed on course for a first global gold of an injury-plagued career.
The American was still well adrift heading into the last 30 metres but somehow found the energy to drive past everyone and become the fourth-fastest man over the distance, behind world record holder Wayde van Niekerk (43.03), Michael Johnson (43.18) and Butch Reynolds (43.29). Hudson-Smith is now fifth.
Fourth-placed Jereem Richards set a Trinidad and Tobago national record of 43.78 and former Olympic champion Kirani James in fifth was also under 44 seconds in 43.87.
"I told you guys I was going to get a gold medal this year and I guess I just showed you I did it," Hall, who switched from the 400 hurdles to focus on the flat, told reporters.
"I know I can win. I knew it today. That’s what I’ve been doing my whole career. I don’t give up. I just grit, I grind. I’ve got determination. Anything I think will get me to that line, I think of it. A lot of hurt, a lot of pain."
Hudson-Smith, 29, has world silver and bronze and now Olympic silver to his name after a long and often troubled career.
"Sometimes the journey is better than the outcome. My time is going to come," he said.
"I can’t describe the feelings, they are so mixed. I was so close to getting the gold. I got an area record and PB.
"I am so grateful to have everyone that came to watch me. It has been a crazy journey. I am just going to keep building."
