William Joseph Parnell MacMillan (March 24, 1881 – December 7, 1957) was a physician and Prince Edward Island politician.

Born in Clermont, MacMillan was a graduate of Prince of Wales College and the McGill University Faculty of Medicine.

After running a medical practice for several years, MacMillan entered politics in 1923 winning a seat in the provincial legislative assembly as a Conservative.

As minister he rebuilt Prince of Wales College and the Provincial Mental Hospital which had both been destroyed by fire.

He also secured a Carnegie Foundation endowment that enabled the government to establish a provincial library system.