Troels Oerting, head of Europol's cybercrime centre, said more criminals are targeting sensitive information on open networks.
"We have seen an increase in the misuse of wifi, in order to steal information, identity or passwords and money from the users who use public or insecure wifi connections," he told BBC Click.
"We should teach users that they should not address sensitive information while being on an open insecure wifi internet.
"They should do this from home where they know actually the wifi and its security, but not if you are in a coffee shop, somewhere you shouldn't access your bank or do all of these things that actually transfer very sensitive information."
Mr Oerting said Europol was helping a number of member states combat attacks on wifi networks.
He warned that the perpetrators often attempted to con people into connecting to a hotspot that resembles public networks, but allows them to capture personal data when it is entered into online accounts.
"Everything that you send through the wifi is potentially at risk," he added.