Porte (BMC) covered the 23.5km from La Tour-du-Pin to Bourgion-Jalieu in a time of 28min 7sec and average speed of 50.14km/h to finish ahead of Katusha-Alpecin’s Tony Martin and Alejandro Valverde (Movistar).
“I didn't expect to win today and certainly not by this much onto the other GC contenders,” Porte said.
“I really worked hard on my time trial this year. I hadn't ridden any time trial for a long time, so I'm really happy with what I did today. Of course, it's always nice to win races. Winning a time trial probably gives even more confidence.
Race leader Thomas De Gendt (Lotto-Soudal) rode an excellent time trial to finish with the ninth best time, enough to give him a reduced 27sec overall lead on Porte.
“I already felt during the recon that I had good legs," De Gendt said. "I had to give it all. I had a good feeling.
"I still have half a minute left onto Richie Porte. I'm really happy with the result I got today and I keep the jersey for at least one more day.”
Valverde is now third overall with Alberto Contador (Trek-Segafredo) and Chris Froome (Sky) trailing in fifth and sixth place.
Amund Groendahl Jansen (LottoNL-Jumbo) set a best time before Cyril Lemoine (Cofidis) established the first performance below thirty minutes at the average speed of 47.36km/h.
Daryl Impey (Orica-Scott) was first to ride above 48km/h before Chad Haga (Sunweb) bettered his time. The next provisional best performance was the one set by Martin, who was close to 50km/h (49.79km/h).
After recording earlier victories in the Santos Tour Down Under, Tour de Romandie and queen stage of Paris-Nice this season, the result places Porte firmly among the 2017 Tour de France contenders.
“I’m happy with where I am at so close to the Tour de France,” Porte said. “With a bit of luck, it can be a good Tour de France for me this year.
“The team has put a lot of faith in me. I'll be the sole leader and I have a great team behind me. Whatever happens, I'll go to the Tour with a fantastic form and a great team to support me.”