Iran summoned the ambassadors of Saudi Arabia and Switzerland, which represents US interests in the Islamic republic, as well as Bahrain's charge d''affaires, to convey the message, the report said.
"The entrance of Saudi forces in Bahrain will only further complicate the situation and transform an internal crisis into a regional one," Hossein Amir Abdolahian, head of the ministry's Persian Gulf and Middle East Affairs department, told Saudi envoy Mohammad al-Kalabi.
Riyadh on Monday sent troops to Bahrain to help the regime control Shiite-led pro-democracy opposition protesters, who have said any foreign force would be treated as an invading army.
Forces from the United Arab Emirates were also present, and Bahraini television showed convoys of unmarked, desert-brown armoured vehicles crossing from Saudi's Eastern Province into Bahrain, home of the US Fifth Fleet.
Iran also protested against the illegal US action in supporting the intervention of foreign troops in minority Sunni-ruled Bahrain, the state TV website quoted the ministry as saying in a separate report.
"The US support for military intervention is contrary to international laws... Thus the Islamic Republic of Iran considers America responsible for the dangerous consequences of this illegal action," an unidentified Iranian diplomat told Swiss ambassador Livia Leu Agosti, the report said.
The main Shiite opposition alliance says it wants a constitutional monarchy, but more radical Shiite groups have said they are protesting to topple the Sunni dynasty that has ruled Bahrain for more than 200 years.
Bahraini charged affaires Jafar Ahmad Ali Hubail was told of Iran's "serious concern" over the situation in the Gulf state.
"The action of foreign forces, no matter what their motives or form, is unacceptable and will only further complicate the situation in Bahrain," an unidentified Iranian diplomat told Hubail, the website said.
Protests erupted in Shiite-majority Bahrain on Feb 14 and several people have died in consequent crackdowns.