The Defence Secretary said appearing on the TV station was “not something I would advise”.
Former Scottish First Minister Alex Salmond hosts a programme on the channel, formerly known as Russia Today.
Mr Salmond has repeatedly defended his connection to the broadcaster.
Speaking last month about The Alex Salmond Show, he declared he would not be “bullied off air” by his critics over his controversial TV programme.
In an interview with The House magazine, Mr Williamson intervened in the row over Moscow-backed TV station RT, which is being investigated by regulator Ofcom.
Asked about MPs appearing on RT, Mr Williamson said: “I can’t understand why people would be wishing to go on what is effectively a Russian propaganda channel that’s obviously propagating the lines of the Kremlin.
“Ultimately people have to make their own choices, but it’s certainly not something that I would advise. “
Ofcom said last month it had launched seven investigations into “the due impartiality of news and current affairs programmes on the RT news channel”.
Mr Williamson called on Vladimir Putin’s Russia to put pressure on Bashar Assad’s regime to end the bloodshed in Syria.
He defended the “clinical” strikes carried out by the RAF against Assad’s chemical weapons capabilities, saying the raids by the UK, US and France was “sending the message that actually actions have consequences”.
He added: “The future of Syria is not going to be won by anyone through the violence and the fighting that we’ve seen in Syria. It can only be through a negotiated settlement.
“What we really need to see happen, you need to see players such as Russia, you need to see players such as Iran who have a great deal of influence on the Syrian regime, start to bring pressure to bear to say ‘actually, a solution has to be found here’.”
In a separate interview with The House, Lord Richards said the risk of failing to increase defence spending risked encouraging opponents to “call our bluff”.
“It’s got to be substantial amounts of money that the Government must spend on the armed forces over the next ten years if they’re to provide or achieve the influence on a day-to-day basis that we need in the post-Brexit era, and secondly, if they are to equip themselves efficiently and effectively in battle, which is our ultimate requirement of them,” he said.
“If there is any doubt about that, then the chances of conflict are increased because our opponents will call our bluff or seek to call our bluff.
“Some of what I’m seeing suggests that we are getting perilously close to that.”
Lord Richards said he was pleased with Mr Williamson’s efforts so far, “but the jury is out”.
“He loses nothing by doing his job, which is to push the case for more spending on the armed forces,” he said.
“It’s whether or not he succeeds that will determine one’s judgment. And so, my worry is that, despite his strong support, there is essentially a lack of political support within Government for defence and the armed forces.”
A spokesperson for RT said: “Continuous baseless attacks on independent voices - be it RT itself, or the MPs and other public figures who choose to appear on our platforms - are precisely what is causing the British and international public alike to lose faith in the values that have long been preached to them and the institutions that are supposed to serve them, not to mention hinder any hope of constructive dialogue and mutual understanding.”