Stan Lee needed a real-life superhero to find and return his lost dog. Enter the immortal football legend, Jim Brown, who proved not all heroes wear capes.

Sources close to Stan tell TMZ ... the comic book legend's Mini Pomeranian, Charlotte, went missing last week, though it's unclear how she got loose. In any case, Stan's peeps canvassed the Hollywood Hills neighborhood where he lives and his business partner Keya Morgan posted "Lost Dog" flyers everywhere.

A few days passed, but the posters garnered zero leads ... leaving Stan resigned to the idea Charlotte was gone for good. But, not so fast.

We're told the NFL Hall of Famer and his wife, Monique -- who live about a mile away from Stan -- found Charlotte in their yard ... scared with no collar. The couple sprung into action ... first firing off an email to a bunch of their neighbors, but that fetched no clues.

The Browns were undeterred, and brought Charlotte to a vet, who found a microchip planted near her neck, but, again, no luck there either because the chip wasn't registered.

The Browns then went old school and canvassed several neighborhoods and BAM ... they saw the posters, called the number and just like that, Charlotte was reunited with Stan.