Achieving global carbon neutrality by 2050 has become a crucial agenda, prompting concerns over energy consumption and carbon emissions in the buildings sector. Particular attention has been drawn to existing buildings that have been in use for many years with poor energy efficiency. Retrofitting to enhance the energy efficiency of these buildings is one of the biggest opportunities the world has to reduce carbon emissions. However, existing building energy efficiency retrofitting (EBEER) faces several challenges that need to be addressed to promote the wider implementation of EBEER. While many studies exist on the challenges, a thorough review of now-available literature is still lacking. This study aims to conduct a thorough review of EBEER challenges. Forty-four relevant studies were identified from Scopus and systematically reviewed. A total of 49 challenges were identified and categorized into technical, financial, institutional, social, environmental, regulatory, and other challenges. Results further indicated that the top three challenges to EBEER are high investment cost, lack of access to finance, and insufficient awareness and knowledge of energy retrofitting. The findings of this study contribute to the understanding of EBEER challenges and may be useful to policymakers and practitioners in developing suitable policies and strategies to address the challenges and promote EBEER.