Is depression the leading cause of suicide?

Yes, actually after a couple of days or weeks of depression it can lead to that person doing suicide. If you see a sign of depression in someone, please tell them you care for them and talk with them to make them feel better. If that doesn't work then ask them whats wrong because that will make that person know that someone cares and knows how they feel.

Further to the above:

"Depression" is a psychiatric diagnosis. Only a fraction of individuals who 'feel a bit down' or 'feel sad' and so on are ever diagnosed as clinically depressed. So although symptoms of depression are often acknowledged (after the event) in those who tragically die by suicide, such symptoms alone rarely lead to suicide.

Edwin Shneidman identified that psychological pain ("perturbation") and access to the means to cause fatal injury to self ("lethality") are invariably present in suicide. It's worth reading widely about this most complex subject where simple, plausible answers to difficult questions such as yours are - almost always - wrong.