Transidentity and autism frequently co-occur. Previous reviews have focused mainly on frequencies. Here, we conducted a systematic review to condense all the studies and themes on this co-occurrence and to offer a global view. We followed the PRISMA method and selected 77 articles (including 59 clinical studies) in April 2022. We found 5 main themes (sex ratio, theories, sexual orientation, clinical and social consequences, and care implications) in addition to frequencies. Many theories have been proposed to explain the co-occurrence. One posits that social difficulties related to autism would lead to less identification with gender norms and less pressure to conform to these norms, allowing for greater gender diversity in people with autism. Given their difficulties with social interactions and communication, the announcement of one's transidentity to one's social group is often discredited, increasing the risk of suffering and delayed care. Many reports reaffirm the importance of providing specialised care for transgender people with autism. Autism is not a contraindication for gender-affirming treatment. However, some cognitive specificities can affect the planning of care, and transgender people with autism are at high risk of discrimination and harassment. We conclude that there is a need to raise awareness about gender and autism.