It said the interview, recorded on 18 March and broadcast on London Live on 8 April, "included potentially harmful content about the coronavirus pandemic".
While not mentioning 5G by name, Icke referred to an "electro-magnetic, technologically generated soup of radiation toxicity" that he claimed had damaged old people's immune systems.
He also claimed that official health advice aimed at reducing the spread of the virus were being implemented to further the ambitions of a clandestine "cult", rather than to protect public health.
Ofcom said it was "particularly concerned" by Icke "casting doubt on the motives behind official health advice to protect the public from the virus".
"These claims went largely unchallenged during the 80-minute interview and were made without the support of any scientific or other evidence."