The computer failures prompted immediate speculation of a hacking attack from communist neighbour North Korea.
The cause of the network shutdowns was under investigation and whether North Korea was behind them remained undetermined, a police spokesman said.
The communications regulator, the Korean Communications Commission, said hacking of an unknown origin was suspected as the cyberalert level was raised to level three on a five-level scale.
Broadcasters KBS, MBC and YTN as well as Shinhan, Nonghyup and Jeju banks and two insurance companies reported that their computers froze about 2 pm (0500 GMT), the Yonhap News Agency reported, citing police.
Woori Bank came under an apparent hacking attack at the same time but it was able to defend its networks, Yonhap said.//DPA