The Los Angeles Galaxy midfielder spent four weeks with the Gunners in a bid to retain his fitness following the conclusion of the Major League Soccer season.
The 32-year-old has not played a competitive game since England's defeat against Croatia in November but was determined to prove his fitness to Fabio Capello as he prepared to name his first squad as national team manager
for next week's game with Switzerland.
However, Beckham was the headline omission from the England squad with Capello citing the former national team captain's lack of match fitness as he sought to win his 100th cap.
"He worked with us for four weeks," Wenger told Arsenal TV Online.
"We gave him the first two weeks exclusively on fitness work with (fitness coach) Tony Colbert. ... After (that) he was with us in and out but in the third week he was a lot with us and we were all amazed how good he was."
Wenger also paid tribute to the fight and resilience Beckham has shown throughout his career which convinces the Gunners chief that he will return to the England team.
"First of all Capello picks on the performances - that is what I rate and what I pick is right," Wenger said. "I believe that what is in Beckham's mind is that I would like to get the 100th cap on performance and on merit not on
"He has worked extremely hard, that this guy will not give up and will continue to fight.
"The second thing I believe as well is that it would be unfair to (him) to give Beckham the 100th cap just because you have done well, let's give you one.
This guy deserves a lot of credit, every day he was first out, last in and give him competitive games with L.A. Galaxy and he will fight for his 100th cap."