Ariana Grande Gives NYC Marathon Runner Surprise of a Lifetime

Ariana Grande is spreading a little extra love in the Big Apple ... teaming up with boyfriend Ricky Alvarez to give one New York City Marathon runner the surprise of a lifetime.

The couple was spotted cheering on runners from the sidelines Sunday when an influencer named Ella Schweiger approached them with an unusual request ... write a message of encouragement for a complete stranger who had just tackled the grueling 26.2-mile race.

Ariana was more than happy to oblige ... grabbing a pen and writing "You did it!" alongside some hearts. She signed the sweet note, "Ari + Ricky."

Ella shared video of the encounter on Instagram ... showing the moment she approached the couple before delivering the message to a runner named Ellie, who was absolutely floored to learn who had written it.

And get this ... Ellie revealed she'd actually been listening to Ariana's music during the race, saying one of her songs helped push her through a particularly brutal stretch around mile 24.

The whole thing was part of Ella's project, "Ella's Envelope," which collects uplifting messages from strangers and delivers them to runners ... proving a few kind words can go a long way.

As for Ariana and Ricky ... the two rekindled their romance earlier this year, nearly a decade after their first breakup, and made things Instagram official in August.