Discounted games will be available until December 3 at 10am PST / 1pm EST / 6pm UK.
Valve has also opened nominations for the 2019 Steam Awards, which can be voted for by players during the autumn sale period.
It was reported this week that Valve has discontinued its Steam Controller peripheral, after selling off its remaining stock in a $5 fire sale.
Valve has shifted its hardware focus to VR in recent years. In 2016 it launched Vive in partnership with HTC, then earlier this year the company announced its own VR headset, Valve Index.
Perhaps most significantly, earlier this month Valve announced its flagship VR title Half-Life: Alyx, which will come free with all Valve Index sales.