Is the daughter of my cousin considered a Mahram (non-marriageable) to me?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The daughter of one's cousin is not considered a Mahram to him, as she is not among those who were mentioned as a man's Mahram women in Surat An-Nisaa’, where Allaah The Exalted Says (what means): {Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives' mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in (i.e. have consummated marriage with). But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allaah is Ever Forgiving and Merciful.} [Quran 4:23]

Consequently, the scholars unanimously agreed that it is permissible to marry the daughter of one's cousin, provided that there is no other impediment under Sharee‘ah (Islamic law) from marrying her, such as suckling.

We draw the attention of the questioner to two things:

First: He must not be ignorant of this type of religious ruling as this is one of the essentials that every Muslim must know. So, he must hasten to learn and understand the matters pertaining to his religion, since, Ibn Mas‘ood narrated, that the Prophet, sallallaahu ‘alayhi wa sallam, said: “Seeking knowledge is an obligation upon every Muslim (male or female).” [Al-Tabaraani, Al-Albaani: Saheeh]

Second: He must be very careful regarding intermixing with non-Mahram (marriageable) women whether they are his cousins or others.

Allaah Knows best.