Background and Objectives: The human gut microbiome is essential for the health of the host and is affected by antibiotics and coronavirus disease 2019 (COVID-19). The gut microbiome is recognized as a contributing factor in the development of ulcerative colitis. Specific vitamins and probiotics have been demonstrated to positively influence the microbiome by enhancing the prevalence of expected beneficial microorganisms. Materials and Methods: Forty-nine ulcerative colitis (UC) outpatients from Riga East Clinical University Hospital were enrolled in this cross-sectional study from June 2021 to December 2021. All patients were divided into groups based on history of COVID-19 (COVID-19 positive vs. COVID-19 negative) in the last six months. Information about antibiotic, probiotic, and vitamin intake were outlined, and faecal samples were collected. The MetaPhlAn v.2.6.0 tool was used for the taxonomic classification of the gut microbiome metagenome data. Statistical analysis was performed using R 4.2.1. Results: Of the 49 patients enrolled, 31 (63%) were male and 18 (37%) were female. Coronavirus disease 2019 was found in 14 (28.6%) patients in the last 6 months. Verrucomicrobia was statistically significantly lower in the COVID-19 positive group (M = 0.05; SD = 0.11) compared to the COVID-19 negative group (M = 0.5; SD = 1.22), p = 0.03. Antibiotic non-users had more Firmicutes in their microbiome than antibiotic users (p = 0.008). The most used vitamin supplement was vitamin D (N = 18), fifteen (42.9%) of the patients were COVID-19 negative and 3 (21.4%) were COVID-19 positive over the last six months (p > 0.05). Vitamin C users had more Firmicutes in their gut microbiome compared to non-users (Md = 72.8 [IQR: 66.6; 78.7] vs. Md = 60.1 [IQR: 42.4; 67.7]), p = 0.01. Conclusions: Antibiotic non-users had more Firmicutes than antibiotic users in their gut microbiome. Only vitamin C had statistically significant results; in users, more Firmicutes were observed. A mild course of COVID-19 may not influence ulcerative colitis patients’ gut microbiome.