Daniel John, a local leader of the main opposition party Chadema in Hananasif ward, Kinondoni, Dar es Salaam, was reported to have been kidnapped along with a party supporter, Reginald Mallya, on Monday.
The politician was murdered while the supporter was left for dead on a Dar beach.
US calls for a transparent investigation to hold all perpetrators of violence accountable.
The United States has raised concern over rising political violence in Tanzania after an opposition leader was found beaten to death on Wednesday.
Daniel John, a local leader of the main opposition party Chadema in Hananasif ward, Kinondoni, Dar es Salaam, was reported to have been kidnapped along with a party supporter, Reginald Mallya, on Monday.
According to Chadema chairman Freeman Mbowe, the two were beaten and tortured over their campaign activities for the party's candidate in the area.
A by-election for Kinondoni is slated for February 17.
Mr Mallya survived the attack after being left for dead at a beach where they had been dumped.
"The party in power wants to get everything by force," said Mr Mbowe accusing the ruling Chama Cha Mapinduzi (CCM) party of being behind the attack.
In a statement released on Thursday, the US Embassy in Tanzania said it was concerned about rising "politically-related confrontations and violence".
"We call on all parties to safeguard the peace and security of the democratic process, the country, and the Tanzanian people," the statement read.
In September 2017, a senior opposition lawmaker Tundu Lissu was shot several times at his home in Dodoma. He was rushed to Nairobi where he was in intensive care for several months, before being transferred to a hospital in Brussels where he continues to receive treatment.
"We also join Tanzanians in calling for a transparent investigation to hold all perpetrators of violence accountable in accordance with Tanzanian law," the US said on Thursday.
