A statement from prosecutors said: "The Swiss bank is suspected of having directly, and not via its Belgian subsidiary, approached Belgian clients to convince them to set up constructions aimed at evading taxes."
UBS said it would "defend itself against any unfounded allegations".
It is the latest in a series of probes into malpractice by banks.
The prosecutors' statement said the judicial investigation came after "excellent" help from French authorities, who had been looking at allegations that UBS had been encouraging clients to open accounts in Switzerland to avoid tax between the years 2004-12.
The US has also been looking into whether the bank allowed its US customers to hide their wealth from the Internal Revenue Service (IRS) to avoid paying full tax.
In 2014, Belgian police carried out raids both at the bank and at the homes of UBS Belgium chief executive Marcel Bruehwiler and a client.
Mr Bruehwiler was also charged at the time.
UBS denied that its Belgian division had ever supported tax evasion.
UBS's small Belgian subsidiary, which employed around 60 staff including 20 private bankers, has since been sold to Belgian private bank Puilaetco Dewaay.
Swiss bank giant UBS is being investigated over money laundering and serious organised tax fraud in Belgium.