Mario Mysteries Wiki
This mystery is considered solved.

Mario and Luigi's last name varies depending on the source.

The name "Mario Bros." has long led people to believe that both brothers have the last name "Mario". This is reflected in sources such as the 1993 Super Mario Bros. movie and the Mario Party 2 Prima Games guide; and is endorsed by the Mario Bros.' voice actor, Charles Martinet.

Other Nintendo employees and representatives, including franchise creator Shigeru Miyamoto, used to state that Mario and Luigi have no last name. Miyamoto changed his mind at some point prior to the Super Mario Bros. 30th Anniversary festival in 2015. During this event, he stated that Mario and Luigi both have the last name "Mario".