Lymphedema is a common complication after mastectomy in breast cancer patients. Many treatment options are available, but no treatment results in a complete cure. We report a case of lymphedema that occurred after modified radical mastectomy in a breast cancer patient who showed objective improvement after delayed breast reconstruction with an latissimus dorsi myocutaneous flap. A 41-year-old female patient with left breast cancer had undergone modified radical mastectomy with axillary lymph node dissection and postoperative radiotherapy 12 years previously. Four years after surgery, lymphedema developed and increased in aggravation despite conservative treatment. Eight years after the first operation, the patient underwent delayed breast reconstruction using the extended latissimus dorsi myocutaneous flap method. After reconstruction, the patient's lymphedema symptoms showed dramatic improvement by subjective measures including tissue softness and feeling of lightness, and by objective measures of about 7 mL per a week, resulting in near normal ranges of volume. At a postoperative follow-up after 3 years, no recurrence was observed. Delayed breast reconstruction with extended latissimus dorsi myocutaneous flaps may be helpful to patients with lymphedema after mastectomy. This may be a good option for patients who are worried about the possibility of the occurrence or aggravation of secondary lymphedema.