The cost of a first-class postage stamp is now 49 cents - 3 cents more than before.
Regulators approved the price hike in December, and it went into effect on Sunday.
The last increase for stamps was a year ago, when the cost of sending a letter rose by a penny to 46 cents.
The Postal Service lost $5 billion last year and has been trying to get Congress to let it end Saturday delivery and reduce payments on retiree health benefits.
If you want to avoid paying more for your stamps in the future, you could start buying forever stamps. They are always valid for first-class postage, no matter what you paid for them, even if prices go up later.
(The Associated Press contributed to this report)