Ryanair said in a statement that Bellew would take over as its chief operations officer on December 1.
Ryanair's chief executive Michael O'Leary said Bellew will lead a significant transformation upon appointment.
Airlines (MAS) appears to have been taken by surprise over news that its chief executive officer Peter Bellew will be leaving to rejoin Irish budget airlines RyanAir.
Bellew - who is now the boss of Malaysia Airlines - will officially take up his new position as chief operations officer on 1 December and will be responsible for all of Ryanair's flight operations, ground operations and engineering.
"We are pleased to welcome Peter back to Ryanair". Having worked previously in a senior role in our Operations team, Peter has an unrivalled knowledge of our business model and how we maintain controlled growth.
"I am excited to return home to Ryanair and take up the challenge to grow the operation sustainably".
Current COO Michael Hickey is stepping down at the end of October after Ryanair was forced to make hundreds of cancellations, costing the company millions of euros, because of crewing problems related to a government-mandated change in how it allocates vacation time.
"The turnaround of Malaysia Airlines remains on track and on schedule, as per the MAS Recovery Plan", it said.
It was pointed out in the statement that Bellew had expressed his commitment to Malaysia Airlines when asked by reporters to comment on speculations that he would re-join Ryanair.
But still, Peter Bellew became the second CEO to jump ship after its predecessor Christoph Mueller left before completing a full-term of his contract in 2016.