Websites are the main information source for investors, so the quality of companies’ websites are vital. This study explores the website quality of listed companies in Thailand and draws a comparison between SMEs’ and large firms’ website quality. In total, concrete and overall objective measures of 271 listed companies (136 large firms from the SET market and 135 SMEs from the MAI market) are collected using a website evaluation tool called SEOptimer. Non-parametric statistics are employed for data analysis due to non-normality. Findings reveal the room for improving the website quality for both SMEs and large companies in each industry. SMEs significantly have lower website quality in terms of SEO, performance, social, security, and overall compared to large enterprises. This study presents the research opportunities to explore listed companies in other countries using other tools in the future and guides SMEs and large companies in Thailand to effectively improve their website quality from the reliable measures and practical suggestions.