The tragic flaw of Othello is his overwhelming sense of jealousy that Iago is able to identify and exploit in order to goad him beyond all bounds and sense of reason to act irrationally, to the extent of killing his wife that he loves so much. Of course, arguably what causes Othello's downfall is not his tragic flaw alone, but the addition of having somebody as evil and as cunning who is able to identify that tragic flaw and exploit it for his own nefarious purposes. Othello's downfall is not something that would have happened without the goading of Iago, and this is something that needs to be remembered in any assessment of his character. Whilst he is unreasonably jealous, this jealousy is only something that is made manifest by Iago's exaggeration of potential concerns and his carefully managed treatment of Othello that is designed to frustrate him as much as possible. At times, the tragic flaw of Othello seems more to be that he appears to be a puppet manipulated by the capable hands of Iago than a thinking human being in his own right, and Iago's treatment of Othello is masterful in how he manipulates him. However, Othello cannot be absolved of all responsibility, and this is something that he identifies in his final speech, when he openly acknowledges his fault. Note how he refers to himself in the following lines:
Of one that loved not wisely but too well,
Of one not easily jealous but, being wrought,
Perplexed in the extreme...
Othello's failing in loving "too well" and being insanely jealous as a result is his own "hamartia" or tragic flaw that is part of the path to his tragic downfall. Even though it is not this alone that results in his tragedy, it is this that Iago is able to exploit in order to bring about the death of his master.
Although a great war hero, the middle-aged Othello's flaw is that he lacks confidence in his own ability to hold the much younger Desdemona's love. He lacks faith in the reality of her love because he lacks faith in his own worth as a lover. After all, he is a dark-skinned Moor in a world in which, as he knows, fairer complexions are favored, and he is a warrior, not a wooer. His lack of confidence in himself renders him susceptible to Iago's machinations. Iago knows Othello's weakness and exploits his advantage to the maximum to destroy his enemy. Rather than trust in his wife, Othello is too quick to believe she would be willing to betray him and has, in fact, already betrayed him by sleeping with another man. This leads him to kill her and thus to destroy not only an innocent woman but his own chance at happiness.