Tag-team partner and brother-in-law Bret Hart posts a picture of the duo, telling fans: "I just don't have the words right now."
Neidhart's ginger goatee beard and pink trousers made him instantly recognisable to fans in the 1980s.
"The Hart Foundation" - which featured Neidhart and brother-in-law Bret Hart - won two world tag-team championships.
Neidhart last competed in 1997 when The Hart Foundation was reformed and his daughter, Natalya, is now a WWE star.
Bret Hart tweeted a picture of the pair in their heyday, with the caption: "Stunned and saddened. I just don't have the words right now."
The local sheriff's office said it appeared Neidhart "fell at home, hit his head, and succumbed to his injury", according to US website TMZ Sports.
Born in Tampa, Florida, Neidhart played American football with the Oakland Raiders and Dallas Cowboys before starting his career in the ring.
Brian Lawler, Brickhouse Brown and Nikolai Volkoff died on 29 July at age 46, 57 and 70 respectively, after successful careers across multiple eras in WWE.