Green tea has been proven highly beneficial in the treatment of acne, comparable to a 4% benzoyl peroxide treatment without the same, negative side effects.
There are three ways in which green tea can help to prevent and treat acne.
1. Green Tea Reduces Sebum Production
Acne is often caused by over-production of sebum formed by the sebaceous glands. One of the ways in which green tea helps to treat acne is by reducing sebum production.
The EGCG component of green tea helps to counteract this by interfering with the IGF-I signals which encourage sebum production.
According to a study done by Myung Im, et al. it was “demonstrated that EGCG suppresses IGF-I induced sebocyte proliferation, lipogenesis, and cytokine expression.
Taken together with our supplementary data, this preliminary evidence suggests that EGCG could be an interesting candidate for the treatment of IGF-I aggravated acne.” 
2. Green Tea Has an Anti-Inflammatory Effect
According to Pubmed “The inflammatory stage of acne vulgaris is usually of greatest concern to the patient. A number of morphologically different inflammatory lesions may form that can be painful and unsightly.”
The EGCG in green tea is well known for its antioxidant properties and so may be beneficial in the treatment of acne to help reduce inflammation and stop pro-inflammatory responses.