Below are the means by which one may become a citizen of Nigeria under the 1999 constitution.
Citizenship by Birth
The first means to become a Nigerian citizen, as itemised under the Nigerian Constitution, is by birth. This is seen under section 25.
A person is a citizen of Nigeria by birth if he is born in Nigeria before, or after, the date of independence, having either of his parent or grandparent belonging to an indigenous Nigerian community. In other words, an individual is a citizen of Nigeria by birth if he or she is born in Nigeria, and either of his parent or grandparent is born in Nigeria.
Also, the Section25(c) provides that a person may become a citizen of Nigeria by birth if he or she is born outside Nigeria, but either of whose parents is a citizen of Nigeria. In other words, you are a citizen of Nigeria by birth if you are born outside of Nigeria, but either of your parents is a citizen of Nigeria.
Citizenship by Registration
This means of becoming a Nigerian citizenship is provided for under Section 26 of the Nigerian Constitution, 1999. As provided, becoming a citizen of Nigeria by registration is applied for.
Under the aforementioned section 26, the possibility of applying to become a citizen of Nigeria through registration is available to a woman, who has married a citizen of Nigeria. Also, a person born outside Nigeria, any of whose grandparents is a citizen of Nigeria, may also apply to become a citizen of Nigeria through registration.
From the foregoing, the two categories of people stated hereinbefore may be granted the citizenship of Nigeria by registration upon the satisfaction of certain requirements. The president must be satisfied that the person is of a good character, has shown clear intention of his or her desire to be domiciled in Nigeria, and that he has taken the oath of allegiance prescribed in the seventh scheduled of the 1999 constitution.
Citizenship by Naturalisation
The third means by which a person might become a citizen of Nigeria is stated under Section 27 of Chapter III of the 1999 Constitution as Naturalisation. This provision affords anybody that meets the stipulated requirements to apply to the president for a certificate of Naturalisation.
For a person to obtain the capability to apply for the certificate of naturalization, the president must be satisfied that he or she is a person of full age and capacity. Also, that the person is of good character, has shown clear intention to be domiciled in Nigeria, acceptable to the local community, capable of making useful contribution to the development of Nigeria, and has taken the oath of allegiance.
Moreover, a person applying for the certificate of Naturalisation must have resided in Nigeria for, at least, the fifteen years preceding the date of his application. Or preceding his application, he has lived in Nigeria for 12 months, and a period amounting in aggregate to at least 15 years within the 20 years immediately preceding those 12 months.
Adapted from a post on LawHub NG.