This paper presents a systematic literature review, defines methods for identifying biomarkers that are characteristic of pain, and considers their possible use for assessing pain in mental disorders. The PubMed, Scope, and Cochrane databases were searched for the subject “pain biomarkers in psychiatric disorders” between 2011 and 2021. Two independent researchers searched available databases for full-text, peer-reviewed studies and review publications using the following keywords: pain biomarkers, pain neuroimaging, pain metabolomics, pain and psychiatric disorders, pain electroencephalography (EEG), serum pain biomarkers, saliva biomarkers, and pain diagnosis. Full-text articles, clinical studies, randomized controlled trials, and systematic reviews were included in the search. The PRISMA method was used to review the literature systematically. The literature search identified 283 studies through the initially assumed inclusion and exclusion criteria. In successive selection steps, 11 studies were selected for analysis. There are three main areas of the possible use of biomarkers for clinically assessing pain in psychiatric patients, i.e., neuroimaging, changes in metabolite levels in body fluids, and gene expression changes. Despite significant research advancements, the described biomarkers are in phases of clinical trials for assessing the intensity and occurrence of pain. Discussion: Pain is a significant and disruptive symptom in patients with mental disorders. Recently, studies have proposed new possibilities for pain diagnostics by determining pain biomarkers. Biomarker research is a dynamically growing field of study. We present examples of pain diagnosis based on biomarkers from various neuroimaging methods and blood and urine analyses. The possibility of new, effective techniques gives hope for the correct diagnosis of pain, especially in patients with mental disorders, which would allow for appropriate and adequate therapeutic therapies. However, the possibilities of use in clinical practice are limited to a few methods. Assessment of pain biomarkers in body fluids (serum, saliva, and urine) appears to be the most practical and promising clinical use method. Keywords: pain assessment; pain biomarkers, psychiatric disorders