Letters and parcels weighing more than 50 grams will cost more to post from today.
An Post said that the move is needed to make up for a 25% shortfall in revenue.
Most letters and cards will cost the same. The basic stamp rate is 55 cent.
Anna McHugh from An Post said that there has been a large decrease in the amount of mail being sent.
"The economic situation is hitting us," she said.
"We have been bringing on new revenue streams, and online shopping is very good news for An Post and our parcel business and packet business is doing very well but not enough to make up for what's being lost with the economy struggling as it is."