The hike is the largest increase in consumer postage prices in more than a decade.
It will affect millions of people, even though fewer Americans these days use snail mail to pay bills and keep in touch.
The hike comes after the agency's regulator gave the green light back in December to keep up with inflation and recoup losses incurred during the great recession.
The agency said in November that it lost $5 billion in the latest fiscal year.
That's far less than in recent years, but that's on top of years of losses.
The agency has exhausted a $15 billion loan from taxpayers to make up for shortfalls.