The 30-year-old singer and Trawick met in 2006 and have been dating since 2009.
"This is something they've both wanted for a long time," the insider told the celebrity news Web site. "It's a way to cement their family. They're both extremely happy and can't wait to become man and wife."
Spears hinted the news was true by posting this Twitter message Friday morning: "OMG. Last night Jason surprised me with the one gift I've been waiting for. Can't wait to show you! SO SO SO excited!!!! Xxo"
UsMagazine.com said Trawick proposed Thursday night at a private dinner celebrating his 40th birthday.
Spears has two young sons with her ex-husband Kevin Federline.