Waterhouse made the announcement while performing on stage at the Corona Central Festival in Mexico City on Sunday, videos posted by fans on X, formerly Twitter, showed.
“I'm extra sparkly today because I thought it might distract you from something else that's going on,” she told the audience.
The crowd then saw a baby bump revealed when she flapped open her coat.
“I'm not sure if it's working,” she said as the crowd cheered in excitement.
Pattinson, of Batman and Twilight film fame, and Waterhouse, who has also modelled in addition to appearing in a number of popular films, began dating in 2018.
The couple are known to be extremely private and rarely discuss their relationship with the media.
They live in Los Angeles but are both originally from the UK.