source: Nigeria 1999 constitution

Chapter III
Citizenship
25. (1) The following persons are citizens of Nigeria by birth-namely-
(a) every person born in Nigeria before the date of independence,
either of whose parents or any of whose grandparents belongs or
belonged to a community indigenous to Nigeria;
Provided that a person shall not become a citizen of Nigeria by virtue
of this section if neither of his parents nor any of his grandparents
was born in Nigeria.
(b) every person born in Nigeria after the date of independence either
of whose parents or any of whose grandparents is a citizen of Nigeria;
and
(c) every person born outside Nigeria either of whose parents is a
citizen of Nigeria.
(2) In this section, "the date of independence" means the 1st day of
October 1960.
26. (1) Subject to the provisions of section 28 of this Constitution, a
person to whom the provisions of this section apply may be registered as a
citizen of Nigeria, if the President is satisfied that -
(a) he is a person of good character;
(b) he has shown a clear intention of his desire to be domiciled in
Nigeria; and
(c) he has taken the Oath of Allegiance prescribed in the Seventh
Schedule to this Constitution.
(2) the provisions of this section shall apply to-
(a) any woman who is or has been married to a citizen of Nigeria; or
(b) every person of full age and capacity born outside Nigeria any of
whose grandparents is a citizen of Nigeria.
27. (1) Subject to the provisions of section 28 of this Constitution, any
person who is qualified in accordance with the provisions of this section
may apply to the President for the same of a certificate of naturalisation.
(2) No person shall be qualified to apply for the grant of a certificate
or naturalisation, unless he satisfies the President that -
(a) he is a person of full age and capacity;
(b) he is a person of good character;
(c) he has shown a clear intention of his desire to be domiciled in
Nigeria;
(d) he is, in the opinion of the Governor of the State where he is or
he proposes to be resident, acceptable to the local community in which
he is to live permanently, and has been assimilated into the way of
life of Nigerians in that part of the Federation;
(e) he is a person who has made or is capable of making useful
contribution to the advancement; progress and well-being of Nigeria;
(f) he has taken the Oath of Allegiance prescribed in the Seventh
Schedule to this Constitution; and
(g) he has, immediately preceding the date of his application, either-
(i) resided in Nigeria for a continuous period of fifteen years; or
(ii) resided in Nigeria continuously for a period of twelve months,
and during the period of twenty years immediately preceding that
period of twelve months has resided in Nigeria for periods amounting
in the aggregate to not less than fifteen years.
28. (1) Subject to the other provisions of this section, a person shall
forfeit forthwith his Nigerian citizenship if, not being a citizen of
Nigeria by birth, he acquires or retains the citizenship or nationality of a
country, other than Nigeria, of which he is not a citizen by birth.
(2) Any registration of a person as a citizen of Nigeria or the grant of a
certificate of naturalisation to a person who is a citizen of a country
other than Nigeria at the time of such registration or grant shall, if he
is not a citizen by birth of that other country, be conditional upon
effective renunciation of the citizenship or nationality of that other
country within a period of not more than five months from the date of such
registration or grant.
29. (1) Any citizen of Nigeria of full age who wishes to renounce his
Nigerian citizenship shall make a declaration in the prescribed manner for
the renunciation.
(2) The President shall cause the declaration made under subsection (1) of
this section to be registered and upon such registration, the person who
made the declaration shall cease to be a citizen of Nigeria.
(3) The President may withhold the registration of any declaration made
under subsection (1) of this section if-
(a) the declaration is made during any war in which Nigeria is
physically involved; or
(b) in his opinion, it is otherwise contrary to public policy.
(4) For the purposes of subsection (1) of this section.
(a) "full age" means the age of eighteen years and above;
(b) any woman who is married shall be deemed to be of full age.
30. (1) The President may deprive a person, other than a person who is a
citizen of Nigeria by birth or by registration, of his citizenship, if he is
satisfied that such a person has, within a period of seven years after
becoming naturalised, been sentenced to imprisonment for a term of not less
than three years.
(2) The President shall deprive a person, other than a person who is
citizen of Nigeria by birth, of his citizenship, if he is satisfied from
the records of proceedings of a court of law or other tribunal or after
due inquiry in accordance with regulations made by him, that -
(a) the person has shown himself by act or speech to be disloyal
towards the Federal Republic of Nigeria; or
(b) the person has, during any war in which Nigeria was engaged,
unlawfully traded with the enemy or been engaged in or associated with
any business that was in the opinion of the president carried on in
such a manner as to assist the enemy of Nigeria in that war, or
unlawfully communicated with such enemy to the detriment of or with
intent to cause damage to the interest of Nigeria.
31. For the purposes of this Chapter, a parent or grandparent of a person
shall be deemed to be a citizen of Nigeria if at the time of the birth of
that person such parent or grandparent would have possessed that status by
birth if he had been alive on the date of independence; and in this section,
"the date of independence" has the meaning assigned to it in section 25 (2)
of this Constitution.