كتب : Omar AbdelAziz
The 27-year-old forward was deemed surplus to the requirements of Boro, who seemed to have reached a mutual agreement to offload him.
"I have officially ended my spell with Middlesbrough and I am travelling to Amsterdam tonight to join Ajax," Mido said on Monday.
"I reached an agreement over a permanent transfer to Ajax, rather than a loan move."
Mido was on the verge of re-joining the Amsterdammers on loan last month, but the deal fell through after the latter failed to agree terms with Boro.
Competition
Mido expressed his delight to be back in the Dutch capital under the management of Martin Jol, but admitted that he needs to work hard for a place in the team.
"I have a good relationship with Martin Jol, I spent a fine spell with Ajax before and the fans like me. I hope to rediscover my form there," he added.
"But I know the competition will be tough, with the presence of forwards like Mounir El-Hamdaoui and Luis Suarez."
Ajax gave away a two-goal lead to draw 2-2 at Groningen in the Dutch league opener on Sunday, courtesy of a brace from El-Hamdaoui.
Jol's side host Vitesse Arnhem on Saturday.