The "Keeping Up With the Kardashians" star was photographed enjoying a winter stroll with their two-year-old daughter True in Boston, Massachusetts on Tuesday (22Dec20), but it was the massive pear-shaped rock on her left ring finger which drew fans' attention.
Khloe has yet to comment on the new piece of jewellery.
She and True are in the city to help basketball star Thompson settle into life in Massachusetts, after recently signing a $19 million (£14 million) deal to join the Boston Celtics from the Cleveland Cavaliers.
Khloe was rumoured to have reconciled romantically with the sportsman over the summer, after spending the coronavirus lockdown together - two years after he was first caught cheating on the reality TV regular.
If the engagement news is true, it will be Khloe's second trip down the aisle. She previously married another basketball player, Lamar Odom, in 2009 but the two split in 2013, and they finalised their divorce in 2016.
The engagement rumors came after Khloe was spotted enjoying a dinner date in Boston with Tristan. Days before the date, the basketball player sparked cheating rumors as he was seen grabbing a bite with a woman who was later identified as his estate manager Julia.