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Simple Summary: Formaldehyde is a chemical compound present in many working activities and indoor workplaces. Occupational exposure occurs primarily by inhaling airborne formaldehyde, but it can also be absorbed through the skin or ingested. The International Agency for Research on Cancer (IARC) classified formaldehyde as a Group 1 carcinogen for humans in 2004, based on toxicological data and epidemiological evidence obtained in workplaces, all published before that year. Over the last two decades, many new studies in this field have been published, providing updated findings. The aim of the present systematic review was to synthetize the results of epidemiological studies in occupational settings carried out in the last 20 years and to evaluate whether the IARC classification was confirmed by further studies. Our results show that the evidence of correlation between formaldehyde occupational exposure and the occurrence of cancer is limited. Background: Formaldehyde, classified as a carcinogen in 2004, as of today is widely used in many work activities. From its classification, further studies were performed to evaluate its carcinogenicity. The aim of the systematic review is to update the evidence on occupational exposure to formaldehyde and cancer onset. Methods: The review, in accordance with the PRISMA statement, includes articles in English reporting original results of studies conducted on workers exposed to formaldehyde, considering all types of cancer, published from 1 January 2000 to 30 July 2021 and selected from the Pubmed and Scopus databases. The studies' quality was assessed by the Newcastle–Ottawa Scale. Results: A total of 21 articles were included, conducted in different European, American, and Asian countries. The most investigated occupational areas are those characterized by a deliberate use of formaldehyde. Some studies evaluated all types of cancer, whereas others focused on specific sites such as thyroid and respiratory, lymphohematopoietic, or central nervous systems. The results showed weak associations with lung cancer, nasopharyngeal cancer, leukemia, and non-Hodgkin's lymphoma. Conclusions: The results demonstrate the need for further original studies carried out on representative samples of workers exposed to measured levels of FA. These studies should be designed to reduce the bias due to co-exposure to other carcinogens. [ABSTRACT FROM AUTHOR]