Grayson County is a county in the U.S. state of Texas. As of the 2020 census, its population was 135,543. The county seat is Sherman. The county was founded in 1846 and is named after Peter Wagener Grayson, an attorney general of the Republic of Texas. Grayson County is included in the Sherman-Denison metropolitan statistical area, which is also included in the Dallas-Fort Worth-Arlington, combined statistical area. Located on the state's border with Oklahoma, it is part of the Texoma region, with proximity to Lake Texoma and the Red River.

The earliest known inhabitants of what is now Grayson County were Caddo Amerindian groups, including Tonkawa, Ionis, and Kichai. These groups engaged in agriculture and traded with Spanish and French colonists at trading posts along the Red River. Trading posts were established at Preston Bend on the Red River, Warren, and Pilot Grove during 1836 and 1837.

After the establishment of the Peters Colony in the early 1840s, settlement near the Red River increased. Grayson County was created from Fannin County by the Texas State Legislature on March 17, 1846. The county seat, Sherman, was also designated by the Texas State Legislature.

In the 1850s, trading and marketing at Preston Bend became more important, as agriculture expanded in the county. This was helped by Preston Road, the first trail in the state. It went from Preston Bend to Austin, Texas. More growth occurred after the establishment of Sherman as a station of the Butterfield Overland Mail route in 1856.

Opinions in the county about secession were divided. County residents voted by more than two to one in 1861 against secession, desiring to remain in the Union. The Great Hanging at Gainesville in nearby Cooke County in October 1862 was an attack on dissenters, men who were suspected of resisting conscription and having been Unionists. After 150-200 men were arrested by state troops, the military organized a so-called "Citizens Court", which had no basis in state law. Its jury made up its own rules and convicted and sentenced more than 25 men to death by hanging. Another 14 were lynched outright by a mob without even the cover of a trial. A total of 42 men were killed in the proceedings that month, considered the largest vigilante murders in U.S. history.

Violence continued for a time in Sherman and other towns of North Texas, at times at the hands of Confederate military. E. Junius Foster, the editor of the Patriot newspaper, was murdered in 1862 by Capt. Jim Young, son of Col. William Young, who had been killed in Cooke County. The senior Young had organized the Citizens Court that put so many men to death, and Foster had "applauded" Young's death. When other men were rounded up as suspect Unionists in Sherman, Brig. General James W. Throckmorton intervened and saved all but five who had already been lynched.

Men from Grayson County served the Confederacy at locations in the South. The 11th Texas Cavalry Regiment captured federal forts in the Indian Territory north of the Red River.

Grayson County and much of Texas suffered economic depression in the postwar years during the Reconstruction era, based in part on difficulties in reliance on agriculture in the South, adjustments to free labor, and other problems. The driving of cattle herds north along Preston Road provided needed income for the county during this period.