Thomas landed the 4ft (1.2m) fish on a stretch of the river near his home in Powys.
His father, Louis Macdonald-Ames, said the pike's head "was bigger than Thomas's".
The fish was not weighed but the pair have estimated it could rival the biggest pike ever recorded from the Wye, caught in 1910, which was 37lb.
Thomas, from Three Cocks, near Glasbury, said: "After a while my arms were starting to hurt so I was glad when we got the fish to the bank. It's the biggest fish I have ever caught."
His father, who works for the Wye & Usk Foundation which looks after the river habitat, said: "Several times the pike took off across the river and, at one stage, I thought I might have to take over the fishing rod.
"I'm proud to say he toughed it out and it was amazing to see what was on the end of the line when the fish finally came in close. We knew it was big, but not that big."
A 12-year-old was left reeling with surprise after catching a "monster" pike in the River Wye.