A US embassy spokesman said on Tuesday that the victim could not be identified until the next of kin had been notified.
The dead man worked for the Vinnell Corporation, whose activities include military training for Saudi forces.
The man was killed as he left a clinic on Electricity Street in eastern Riyadh's Gulf district, al-Arabiyya satellite channel said, quoting its correspondent in the kingdom.
Two days ago armed dissidents shot dead an Irish cameraman and gravely wounded a BBC correspondent in Riyadh as they were filming near the home of a slain opposition fighter.
And on 28 and 29 May, four armed dissidents killed 22 people including Westerners in a hostage-taking drama in the eastern oil city of Al-Khobar.
Usama bin Ladin's al-Qaida has been blamed for repeated attacks on compounds housing Westerners and Arabs in Saudi Arabia in the last year in which more than 85 people have been killed.
Saudi dissidents claim there is a widespread insurgency in the kingdom aimed at overthrowing the unelected pro-western monarchy.