Disclosed is a quick-donning mini-jacket with matching left and right segments, either one of which may independently be used to cover and/or protect a baseball pitcher's throwing arm. Upon insertion of the pitcher's arm into either segment, the wearer may experience a snug, comfortable fit of the garment about the perimeter of the wearer's shoulder and arm, by virtue of an inner elastic band. Should the inner elastic band be insufficient for a close and comfortable fit, an adjustment strap is provided, typically of a hook-and-felt fastening mechanism. By adjusting tightness of the strap, the tensile force about the shoulder joint may be increased a comfortable amount. In this manner, a pitcher's arm can be kept warm, relaxed, and relatively free from muscle tightening or tension that might result from a just-completed pitching session.