Adan Bule Mohamed was killed after being shot several times in the head and shoulder in front of his house in the Dharkinley neighbourhood in the south of the capital, they said.
“Masked men killed the parliamentarian and disappeared before the police arrived,” Abdi Ismail, a witness, told AFP.
There was no immediate information on the possible motive for the attack.
Somalia has been wracked by violence since 1991, when it first spiralled into civil war. Since then, several foreign armies — including US forces, UN peacekeepers and a 2006 Ethiopian invasion — have failed to create stability in the anarchic nation.
The 550 Somali deputies were named for the most part in 2004 and their mandate is due to expire on August 20, 2012.
The weak Western-backed government of President Sharif Sheikh Ahmed survives in Mogadishu under the protection of more than 9,000 African Union troops, who have spent four years battling the Al-Shebab Islamist rebels aiming to topple his administration.
Last month, Kenya deployed troops into the Shebab-controlled southern Somalia to battle the Al-Qaeda-inspired rebels it blames for kidnapping foreigners on its soil and conducting cross-border raids.