Better understanding differences associated with antibiotic prescribing for acute sinusitis can help inform antibiotic stewardship strategies. We characterized antibiotic prescribing patterns for acute sinusitis among commercially insured adults and explored differences by patient- and prescriber-level factors. Outpatient encounters among adults aged 18 to 64 years diagnosed with sinusitis between 2016 and 2020 were identified by national administrative claims data. We classified antibiotic agents-first-line (amoxicillin-clavulanate or amoxicillin) and second-line (doxycycline, levofloxacin, or moxifloxacin)-and ≤7-day durations as guideline concordant based on clinical practice guidelines. Modified Poisson regression was used to examine the association between patient- and prescriber-level factors and guideline-concordant antibiotic prescribing. Among 4 689 850 sinusitis encounters, 53% resulted in a guideline-concordant agent, 30% in a guideline-discordant agent, and 17% in no antibiotic prescription. About 75% of first-line agents and 63% of second-line agents were prescribed for >7 days, exceeding the length of therapy recommended by clinical guidelines. Adults with sinusitis living in a rural area were less likely to receive a prescription with guideline-concordant antibiotic selection (adjusted risk ratio [aRR], 0.92; 95% CI, .92-.92) and duration (aRR, 0.77; 95% CI, .76-.77). When compared with encounters in an office setting, urgent care encounters were less likely to result in a prescription with a guideline-concordant duration (aRR, 0.76; 95% CI, .75-.76). Opportunities still exist to optimize antibiotic agent selection and treatment duration for adults with acute sinusitis, especially in rural areas and urgent care settings. Recognizing specific patient- and prescriber-level factors associated with antibiotic prescribing can help inform antibiotic stewardship interventions.