Apolo Ohno -- The Subway Card ... IS A MYTH!!!

Apolo Ohno just blew our minds ... telling TMZ Sports the mythical Subway "Free Subs for Life" card is like Bigfoot ... IT DOESN'T REALLY EXIST!!!

And here's the catch ... he's kinda, sorta right.

The convo was brief, but Ohno -- who's done all sorts of work as a Subway pitchman over the years -- told us the "Eat for Life" thing isn't possible because Subway franchises locations out all over the world.

We did some digging ... and here's what we know ...

There IS a special Subway "card" that Subway gives to V.I.P.s -- but it's not an automatic pass for free food.

Our sources tell us the card is more like a debit card ... and Subway loads it up with "credit" that can be redeemed at any Subway location. When the credit runs down, the V.I.P. can call up the Subway honchos to load up the card again ... if Subway obliges.

So, yeah ... the card is awesome -- but it doesn't automatically cover you for life. And now you know.

Sorry Happy Gilmore.