Jenna Fischer's showing love for her old 'Office' romance ... congratulating John Krasinski on capturing the Sexiest Man Alive crown -- though, she doesn't sound too enthusiastic about it.

The actress -- who played Pam Beesly on "The Office" alongside Krasinski's Jim Halpert for all 9 seasons of the show -- stepped out in L.A. Thursday ... and, a photog asked her how she felt about John's new title.

JF laughs when asked if she ever imagined John would become Sexiest Man Alive when they costarred together on "The Office" ... before congratulating the star and saying "Good for him."

Listen to the clip for yourself ... we're not saying Jenna's not happy for John -- but, she's not falling all over herself to sing his praises.

Of course, Jenna recently said her battle with breast cancer has put what's really important into perspective ... and, we doubt that means the Sexiest Man Alive title.

As you know ... Fischer announced her diagnosis last month, explaining when she went for a routine mammogram, she was diagnosed with an aggressive -- though highly treatable -- form of cancer.

She opened up about it in more detail later in an interview with Hoda Kotb ... diving into how it changed her outlook on life -- where the things that used to annoy her just don't hit as hard anymore.