"It was the farthest left-handed-hit home run I've seen here," right fielder Justin Upton said.
Upton was caught by cameras looking in amazement as the ball landed.
"Statistics say I hit a couple longer," Upton said. "I don't believe it."
Allen's homer carried an estimated 455 feet by ESPN Stats and Information.
"I guess it was a good shot," Allen said after Tuesday's loss. "I'll take it."
D-backs manager Kirk Gibson, who was known to hit a long home run or two back in his day, was as amazed as Upton with the home run.
"Oh, my gosh," he said. "How do they measure that?"