Kind of, but you can only "opt-in" by using so called "secure chats". From their FAQ at https://telegram.org/faq#q-so-how-do-you-encrypt-data:

"We support two layers of secure encryption. Server-client encryption is used in Cloud Chats (private and group chats), Secret Chats use an additional layer of client-client encryption."

It seems like many people (especially users/advocates of telegram) are confused by this, since telegrams marketing sounds like it's fully end-to-end encrypted.