Thomas announced the decision on Sunday via GoDucks.com, the school's athletic website.
"I am officially withdrawing from the University of Oregon to pursue a professional career in the NFL," he said in a prepared statement released by the school. "I want to express my deepest appreciation and thanks to the university and all of my teammates, coaches and fans. I look forward to staying connected to the university and visiting the sidelines as often as possible."
Known for his speed, Thomas finishes his college career with 5,345 career all-purpose yards, trailing only LaMichael James and Kenjon Barner on Oregon's career list. Thomas holds Duck records for kickoff return yards with 1,885 and a 17.1-yard punt return average.
The 5-foot-9, 170-pound Los Angeles native ran for two touchdowns in the 2012 Rose Bowl victory over Wisconsin. The next season in the Fiesta Bowl win over Kansas State, he ran the opening kickoff 94 yards for a touchdown.
Thomas established a reputation as a dual threat last season, running for 701 yards and 11 touchdowns and catching 45 passes for 445 yards and five more scores. He also scored on a kickoff return and a punt return. He was the first Oregon player in 47 years with a touchdown four different ways.
This season he led the Ducks with 338 yards rushing and six touchdowns through the first three games this past season, but he injured his right ankle when he slipped on the opening kickoff of Oregon's victory over California on Sept. 28. He missed the next three games.
Thomas finished the season with 594 yards rushing for eight touchdowns, and he had 22 catches for 249 yards and another score. He averaged 135.3 all-purpose yards a game.
"I will certainly hate to see De'Anthony leave, but I understand and agree with his decision to enter the 2014 NFL draft," running backs coach Gary Campbell told GoDucks.com. "He's had an exciting career at Oregon, and now it's time to move on to the next level. I wish him mega success in his new endeavors."
Thomas also ran track for the Ducks and anchored the 4x100-meter relay that went to the 2012 NCAA Championships. The team ran the event in 39.89 seconds, sixth-fastest in Oregon history.
Thomas is the second Duck to leave school early for this year's NFL draft. Cornerback Terrance Mitchell announced his plans to forgo his final season of eligibility at Oregon on Thursday.
Mitchell led the Ducks with five interceptions and 12 passes defended this season, in giving him seven interceptions and 32 passes defended for his career.
Earlier this season, sophomore quarterback Marcus Mariota and junior center Hroniss Grasu both announced their intention to remain with the Ducks next season.
The No. 10 Ducks finished the season 11-2 and capped it off with a 30-7 victory over Texas in the Alamo Bowl.
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