Cases of poor mental health have been on the increase in the construction industry, with construction professionals frequently encountering stress, anxiety, depression, and substance abuse. Yet, many are unaware or lack sufficient knowledge of the symptoms. The study aims to examine the knowledge level of mental health symptoms among construction professionals. A quantitative research design was adopted using an electronically distributed survey form to professionals working in construction firms in Lagos state. 400 survey forms were distributed using simple random and purposive sampling techniques, while 366 forms were returned and valid for analysis, representing a 91.5% response rate. The data were analysed using mean score and Kruskal–Wallis. On average, the result indicates that the knowledge level of mental health symptoms by construction professionals is moderate. Furthermore, safety managers tend to have a high knowledge level of mental health symptoms compared to other professionals on construction sites. The study thus concludes that while construction professionals have a basic understanding of mental health symptoms, gaps still exist in recognizing and addressing complex conditions. The study recommends that formal education, training, and discussions about mental health should be encouraged and adopted as policy and practice in construction firms.