Talk about a golden touch … TMZ Sports has learned Floyd Mayweather used an 18 karat gold pen — worth $4,000 — to sign his contract for the big Pacquiao fight.

It’s called the Peerless 125 (because there are only 125 in the world) by Cross … and it’s one of the most expensive writing implements in the world.

A spokesperson for Cross says he’s pumped to add Floyd to the list of “noble” clients who use the company’s pens … including Oprah and President Obama, who has actually signed official docs with a Cross.

BTW — Floyd retired the pen after he signed contract … saying it’s now “a part of history.”

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