The games are currently available for purchase on the Google Play store for $9.99 each (North America only), but can only be played on Shield.
In a statement announcing the news, Nvidia said there are now over 300 games on the Google Play store that support Shield.
"Nvidia has done a remarkable job bringing both Half-Life 2 and Portal to Shield," said Valve's vice president of marketing Doug Lombardi.
"We're playing both games here on our Shields and fans of both franchises can expect the same gameplay they've come to love on the PC."
Nvidia recently announced plans to introduce 'GameStream-ready' PCs and routers in an initiative designed to simplify the process of using its wireless game streaming technology, designed to allow low-latency streaming of home PC games to Shield via Wi-Fi.