Rail passengers travelling with Arriva Trains Wales will see their ticket prices rise by 3.3%.
From Tuesday, average rail ticket prices will rise by 3.4% across the UK - the biggest increase in five years.
Protests are planned at some 40 stations, including at Cardiff Central.
The National Union of Rail, Maritime and Transport Workers (RMT) called the rises "eye watering" but the Department for Transport said price rises were capped in line with inflation.
Welsh commuters travelling on Great Western Railway lines will also have to pay 3.1% more for their ticket.
Many annual season tickets prices have increased - such as from Neath to Cardiff, which has gone up by £56 to £1,708.
RMT said its members would be handing out chocolates at the Cardiff protest between 07:00 and 09:00 GMT in a bid to "sweeten the bitter pill" of the fare rises.
Fare increases to regulated fares are calculated using the previous July's Retail Prices Index (RPI) measure of inflation.
Around half of all tickets fall under this category.
RMT General Secretary Mick Cash said: "It's another New Year and yet another hike in fares for passengers. These eye watering increases will make it even harder for workers to get by."
Stephen Joseph, chief executive of the Campaign for Better Transport (CBT), accused the UK Government of choosing to "snub rail passengers" by continuing to raise fares while fuel duty is frozen for a 17th consecutive year.
But the Department for Transport said 97p out of every £1 paid by rail passengers went back into the railway.
"We are investing in the biggest modernisation of our railways since the Victorian times," a spokesman added.