The bill had been passed by the Democrat-controlled Legislature in July and Monday was the deadline for the Republican governor to sign it, The Wall Street Journal reported.
The law prohibits professional counselors from administering "sexual orientation change efforts" to anyone less than the age of 18.
Christie is opposed to gay marriage but supports the state's existing civil unions that give gay couples the same benefits as married couples.
New Jersey is expected to be the first state to have such restrictions in force, said Garden State Equality, the state's main gay advocacy group. A similar law in California is being challenged in the courts.
Christie's office has yet to release documentation of the signing, and state legislators have not yet been briefed on the signing, legislative leaders said.