Does niacin help treat depression?

NO. Nicotinic acid or Niacin is not intended to treat depression and has not been approved in this condition. Niacin is used for treatment of "dyslipidemis or imbalance of cholesterol - it increased "good" HDL cholesterol. It should not be taken without medical consult due to possible liver toxicity.
For many but not all. There are many different causes of depression & no single treatment helps everyone, but many who suffer from depression have found it to be very helpful. More research is needed but it is safe to try. Due to the flush it should be started in low doses & gradually increased. It can also help anxiety and even some cases of schizophrenia. See www.Squidoo.Com/niacin-another-weapon-against-depression.
Only if Deficient. Niacin has been discussed as a potentially helpful treatment for depression. My understanding is that it is not usually of significant benefit for moderate to severe depression. You are most likely to get benefit from Niacin if you are dealing with nutritional deficits.