The periarticular multimodal cocktail injection including morphine is currently commonly used to reduce postoperative pain following total knee arthroplasty (TKA). Despite its analgesic effect, it frequently causes nausea, which is an adverse effect of opioids. It is inconclusive whether the intraoperative injection of periarticular morphine is effective peripherally. The aim of this study was to assess whether the addition of morphine to unilateral periarticular knee injections improves postoperative pain, range of motion, and swelling in patients undergoing simultaneous bilateral TKA. A prospective, single-center, double-blinded, randomized, controlled trial was conducted to assess the local efficacy of adding morphine to intraoperative periarticular anesthesia in patients undergoing simultaneous bilateral TKA. Fifty-three patients undergoing 106 TKAs received an intraoperative periarticular injection in randomly selected one knee with added morphine (0.1 mg/kg) and the other knee without added morphine. The periarticular injection was composed of ropivacaine (a local anesthetic), epinephrine, ketoprofen, and methylprednisolone sodium. Visual analog scale pain scores at rest and on motion, range of motion (ROM), thigh swelling, the Western Ontario and McMaster Universities Osteoarthritis Index score, and adverse outcomes were compared between the 2 knees. There were no statistically significant differences in the visual analog scale score, ROM, thigh girth, Western Ontario and McMaster Universities Osteoarthritis Index score, and adverse events between the 2 sides. Adding morphine to periarticular injections is ineffective locally for relieving pain, reducing swelling, and improving the postoperative ROM.