Fans of the That '70s Show star were left baffled on Thursday night (03Jul14) when he seemed to post a string of bizarre updates.
One tweet included a snap of him and his pop star girlfriend in bed together, with Lovato seductively biting his lip. The image was originally leaked back in April (14).
Another included a screen grab of cartoon The Powerpuff Girls, with a large character being labelled as 'Demi', and an accompanying caption: "Shes fat as f**kk."
After the updates, Valderrama logged in himself to explain to fans that an imposter had broken into his account.
He wrote, "Attention Everyone! Someone just hacked my twitter account!!!!!"
His page has since been removed.