Nyquist -- Pulled from Belmont ... Medical Complications

Nyquist -- the winner of the Kentucky Derby -- has been pulled from the Belmont Stakes ... after complications with his health, TMZ Sports has learned.

We spoke with Nyquist's assistant trainer, Jack Sisterson, who tells us ... Nyquist has an elevated white blood cell count, which can sometimes indicate an infection.

Nyquist was already battling a fever -- but Sisterson hoped Nyquist would overcome it in time for the Belmont Stakes next month.

Now, with this new complication, Sisterson says it's better to play it safe -- and not push Nyquist too hard.

"We just want to do right by Nyquist right now. We want to make sure he'll be back to 100% in the next few days."

As for his future -- "He's definitely going to race again this summer."