The Joseph Rowntree Foundation said the number of children in working families in poverty rose by 22,000 between 2003 and 2013.
Chief executive Julia Unwin said action to help pensioners was balanced by a "worrying rise" in working people struggling to "make ends meet".
The UK government said its plan for a national living wage would help people.
"Work is the best route out of poverty and employment in Wales is at record levels," a spokesman said.
A Welsh government spokesman added: "Despite recent improvements to the Welsh economy, we do, however, recognise in-work poverty is a growing issue and are working hard to support low-income households and deliver more well-paid jobs for the Welsh economy.
"We are helping more adults access full-time employment, supporting second-earners into work and are also improving people's skills to enable them to progress in the workplace."
People are classed as being in poverty if their household earns or receives less than 60% of the national average income of around Â£450 a week, adjusted for family size.
Low pay and scarce hours are pushing working families and young people into poverty in Wales, a new report claims.