Adele in Tears After Spotting Celine Dion at Las Vegas Residency

Adele had a tearful interlude at her Vegas residency Saturday night ... when she spotted a fan in the audience ... and that would be Celine Dion.

Adele was singing "When We Were Young" at a packed arena at the Colosseum at Caesars, when she eyeballed the famous Canadian in the crowd.

Adele beelined it over to the "My Heart Will Go On" singer ... the 2 embraced as Celine clasped Adele's face. Tears flowed and Kleenex appeared.

It just took a beat for all this to happen and then Adele got back to business. But there was more to come.

Adele properly intro'd Celine, and the crowd went wild, giving her a standing O.

As you know, Celine's been struggling with Stiff Person's Syndrome for several years and has pulled back on performing, although she made a triumphant return during the Paris Olympics.