"They can have similarities in their technical skills and talent, and it is hallucinating for football to have them both present, but Messi is simply the best player in the world," Beckham told Telam.
"He [Messi] is alone in his class as a player, it is impossible that there is another like him," the 37-year-old midfielder told Telam. "He, like Cristiano Ronaldo, who is not at his level, are both above the rest."
The former England captain won Champions League in 1999 with Manchester United. He joined PSG at the end of January after five years with the L.A. Galaxy in the MLS.
"We were leading before Messi came in, and once he came in, Barcelona scored," Beckham said.
Messi came off the bench to help Barcelona fight back for a 1-1 draw with Paris Saint-Germain on Wednesday and reach the Champions League semifinals for a sixth year in a row. Barcelona will face Bayern Munich in the semifinals.
"Although I enjoy playing at this level at my age, I do not like to lose," Beckham told Telam. "Our team played well. In both matches we did things we should be proud of. We didn't lose against Barcelona, and that is something that should motivate us for the next season."
Barcelona advanced on the away-goals rule after its 2-2 draw in Paris last week meant the contest finished 3-3 on aggregate, but not without showing just how much it depends on Messi against major opponents.
Messi, who had injured his hamstring a week earlier, was cleared to play only an hour before kickoff. He went on in the 62nd minute with his team losing 1-0 after Javier Pastore's opener.
It took him nine minutes to start the decisive move that Pedro Rodriguez finished off to keep Barcelona in the hunt for its fourth Champions League title in eight seasons.