The airline claims this brings its approach into alignment with standard practice among most European carriers
Ryanair will allow children to sit next to their parents on flights for free.
An investigation by the Competition and Markets Authority (CMA) into the budget carrier's practices was launched, pressuring the company into changing the policy.
The change comes after the CMA began examining whether the airline's previous approach to seat reservations breached consumer protection legislation.
Under the revised system, adults flying with children who opt not to purchase reserved seating will receive notification of their complimentary seat assignment once they have completed check-in.
The airline claims this brings its approach into alignment with standard practice among most European carriers.
The airline's previous policy required adults travelling with children to pay a single reserved seat fee, which then allowed them to select adjacent seats for up to four children at no additional charge.
According to the CMA, this arrangement typically resulted in charges of £8 per direction of travel when the regulator announced its investigation earlier this month.
Ryanair has described the adjustment as a "minor policy tweak" that took effect today.
The complimentary parent seats will be situated towards the rear of the aircraft.
The carrier anticipates no impact on its revenue from this modification.
Michael O'Leary, the airline's chief executive, said Ryanair would "reluctantly adjust to this industry standard" while maintaining that its longstanding approach had fully complied with legal requirements and provided families with "certainty".
He criticised the regulator for targeting what he described as a policy "universally embraced by consumers as the most progressive and transparent in Europe".
Mr O'Leary added: "Instead of promoting competitiveness and lower fares for consumers, the CMA is on a mission to force Ryanair to adopt the less transparent and less consumer-friendly family seating policy applied by most other airlines just because it's the industry standard.
A CMA spokesman responded: "Ryanair claims its seating policy now complies with the law, and we'll test that thoroughly.
"If true, it's a win for families who will no longer have to pay to sit with their children and it shows the impact our new powers are having."
They added: "But it doesn't change the fact families have been paying for 'mandatory family seats'.
Consumer group Which?, which had previously flagged the airline's seating charges to the regulator, welcomed the development but expressed concern about the circumstances.
Rory Boland, editor of Which? Travel, said: "It's clear Ryanair is unhappy about being dragged into doing the right thing, so Which? will be monitoring the implications of this policy and whether all parents are seated next to their children without charge over the next few months."