The 15th Massachusetts Infantry it served as a member of Harrow’s Brigade in Gibbon’s Division of the Second Corps, Army of the Potomac, a Fighting 300 Regiment. The regiment was engaged in many battles from Ball's Bluff to Petersburg, and suffered the tenth highest fatality rate amongst Federal regiments. The regiment was composed almost entirely of men from Worcester County, and was mustered in on July 12, 1861. The 15th was commanded by Colonel George H. Ward (1826-1863). Ward was a farmer from Worcester. He was wounded at Ball’s Bluff where he lost his left foot. On July 2, 1863, during the Battle of Gettysburg, Ward's regiment was heavily engaged on the Union left-center along Cemetery Ridge and Colonel Ward, along with 36 others, was mortally wounded. He died the following day and is buried in the Rural Cemetery, Worcester. Under his brief command, 304 men from the 15th were engaged at Gettysburg and among them, 23 were killed, 97 were wounded and 28 went missing. All totaled, 4 officers and 227 enlisted men were killed or fatally wounded during the course of the war.
The 15th Massachusetts Infantry Monument is located on the left or northwest side of 
Hancock Avenue (RD310) if traveling north along the road at the beginning of an area called The Angle. This monument is the fourth sculpture in a string of monuments that extend north across the green field to the United States Regulars Monument (MN235) and beyond. All totaled, there are about 1080 feet of green fields with a line of monumentation spread across it representing the line of union regiments who defended Cemetery Ridge at The Angle against Longstreet's assault also referred to as Pickett's Charge on July 3, 1863, the final day of the Great Battle. The 1st Minnesota Infantry Monument (MN239) is 88 feet to the south or left of this monument is facing the west. Next is the 82nd New York Infantry Monument (MN239) and then the 20th New York State Militia (80th Regiment) Marker (MN798). Finally, the 149th Pennsylvania Infantry Monument (MN248) is directly across the road, 248 feet southeast of this position. 
Parking is plentiful and is available road-side at intermittently enlarged shoulder cut-outs, usually marked with white striping. Be sure to keep vehicles off the grass or you will be ticketed by park police. I visited this monument on Tuesday, August 7, 2012 at approximately 3:38 P.M. I was at an elevation of 583 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
The Draw the Sword site helped out by the NPS narrative and the SIRIS site offers the following description: Vertical marker on a square base contains a full-length relief of a uniformed Union soldier. He holds his rifle diagonally across his chest in both hands and wears a cap and moustache. A trefoil appears above the relief at the top of the marker. Monument is a 2.6 foot square granite shaft with a gable top that has an inscription of the west face and set on a four foot square smooth base. The shaft has a recessed bronze relief of a standing infantryman with a gun. Overall height is 7.6 feet. Flanking markers are 1.6×8 foot with gable tops that have inscriptions. The monument originally stood east of the Copse of Trees, near the present location of the High Water Mark monument. It was moved by the Battlefield Memorial Association in the late 1880s to the position held by the Massachusetts troops on July 3, 1863.
The monument was dedicated June 2, 1886 by the Commonwealth of Massachusetts. The monument is composed of granite with bronze relief and has the following dimensions: The sculpture is approximately 54 x 30 x 30 inches and the base is 2 x 4 x4 feet.
One SIRIS site speculates W.P. Dolly might have been the sculptor but I have no clue upon what that information is based. I know for sure the sculpture was fabricated by the Boston Marble & Granite Company out of Worcester, Massachusetts as I saw and photographed their stamp on the monument. They worked on many Massachusetts monuments @ Gettysburg. There are the briefest of inscriptions on the front and reverse sides of the monument which read:
(Front): 15th Mass.
(Back):15th Regt. Mass. Vols.
1st Brig. 2nd Div. 2nd Corps.
July 3. 1863.
The 15th Massachusetts Infantry Monument is a contributing feature to the Gettysburg National Military Park Historic District which is nationally significant under NR Criteria A, B, C & D. Areas of Significance: Military, Politics/Government, Landscape Architecture, Conservation, Archeology-Historic. Period of Significance: 1863-1938. The original National Register Nomination was approved by the Keeper March 19, 1975. An update to this nomination was approved by the Keeper on January 23, 2004. The monument is identified as structure number MN203-B.
From the Nomination Form:
1 of 25 mns in Park honoring MA commands in Gettysburg Campaign. Marks original position of 15 MA on July 3, 1863. Located on W side of Hancock Ave & S of Angle & Copse of Trees.
Short Physical Description:
MN & 2 Flank Markers. Smooth base 4' sq. Shaft: 2'6"sq. w/recessed bronze relief of standing infantryman w/ gun. Gable top w/ inscription on W face. All 7'6"H. Flank markers, 1'6"x8"x2', gable tops w/ inscriptions on polished face.
Long Physical Description:
Monument that has two flanking markers. Monument is a 2.6 foot square granite shaft with a gable top that has an inscription of the west face and set on a four foot square smooth base. The shaft has a recessed bronze relief of a standing infantryman with a gun. Overall height is 7.6 feet. Flanking markers are 1.6x8 foot with gable tops that have inscriptions. Located on the west side of Hancock Avenue south of the Copse of Trees.
My Sources
1.
NRHP Nomination Form
2.
SIRIS
3.
Stone Sentinels
4.
Virtual Gettysburg
5.
Draw the Sword
6.
Historical Marker Database
7.
Wikipedia