Aim To investigate (1) the association of lifestyle changes and living and working conditions with glycemic control and (2) whether treatment was intensified appropriately in patients with diabetes under the first COVID‐19 state of emergency in Japan. Materials and Methods A total of 321 participants were included. Participants completed a questionnaire regarding lifestyle changes, including diet, physical activity, and living and working conditions during the COVID‐19 pandemic. The change in hemoglobin A1c (HbA1c) levels was estimated before (June 1, 2019 to August 31, 2019) and during (June 1, 2020 to August 31, 2020) the pandemic. Factors associated with changes in HbA1c levels were examined by multiple linear regression analysis. The proportion of patients who received treatment intensification for diabetes was compared between before and during the pandemic. Results There was no significant change in HbA1c levels before the pandemic and during the pandemic (7.13 ± 0.98% vs 7.18 ± 1.01%, P = 0.186). Teleworking (estimate 0.206, P = 0.004) and living with a dog (estimate −0.149, P = 0.038) were significantly associated with changes in HbA1c levels after adjusting for covariates. There was no significant difference in the proportion of patients who received treatment intensification for diabetes during the pandemic and before the pandemic in either the elderly or non‐elderly patients. Conclusions Overall glycemic control did not worsen during the pandemic. Nonetheless, environmental factors, including telework, were found to influence glycemic control in patients with diabetes. Further studies are needed to clarify whether the COVID‐19 pandemic could affect treatment intensification for diabetes. The study investigated (1) the association of lifestyle changes and living and working conditions with glycemic control and (2) whether treatment was intensified appropriately in diabetes patients under the first COVID‐19 state of emergency in Japan. Teleworking and living with a dog were positively and negatively associated with changes in HbA1c levels, respectively. The proportion of patients who received treatment intensification for diabetes during the pandemic was slightly lower than that before the pandemic, but the difference did not reach statistical significance.