Yes, if your partner meets all of these tests. If the child is under 19 and doesn't work, it really comes down to providing more than 50% of the child's financial support. ("You" refers to your partner and "they" is the child.)
1) You provided more than half
of their financial support. More info
2) They made less than $4,050 in gross income
during 2016 unless they are a
qualifying child (under 19 OR a full time student under 24
OR permanently disabled)
3) They live with you or they are related to
you.
4) They are a U.S. citizen, resident alien,
national, or a Canadian or Mexican resident.
5) They aren't (or won't be) claimed as a
dependent by someone else.
6) They aren’t filing a joint return with
their spouse.
7) You are not being claimed as a dependent on
someone else's return.
