When American teenager Dustin Snyder was told his cancer had returned and he had only weeks to live, he proposed to his high school sweetheart.

Mr Snyder has a rare form of cancer called synovial sarcoma, The Washington Post reported.

Coming to terms with the news he had only a handful of weeks to live, the 19-year-old asked Sierra Siverio to marry him.

"After my first surgery, when she was there for me in the hospital for 10 days straight — [she] did never leave — that's when I knew she was the one," he told the BBC.

The impromptu wedding-planning started off with a Facebook post in which Mr Snyder's sister asked if anyone had a tuxedo and wedding dress to lend.

But the call-out quickly escalated to a $40,000 ($AU49,500) crowdfunding campaign — among generous offers were venues, food, flower arrangements and music.

Dustin Snyder said Sierra Siverio was "the one". ( Facebook: Nicole Jordan Roberts )

Last Sunday, the two tied the knot near their home in Valrico, Florida.

"I wouldn't want to be standing here with anyone else," Mr Snyder told the bride at the ceremony.

Ms Siverio said of the "amazing" day: "I was nervous but I was excited because I got to marry my best friend."