Small counties in the coastal Southeast had some of the largest population gains between mid-2024 and mid-2025 in estimates being released Thursday by the U.S. Census Bureau, mostly because of people moving from larger areas.

Jasper County, South Carolina, where there’s a building boom taking advantage of the popularity of nearby Hilton Head, was the fastest-growing county in the nation percentagewise, growing 6 percent in the year to 38,533 people. It grew even faster the previous year, 6.9 percent, but another county elsewhere grew slightly faster that year — Mellette County in South Dakota.

Jasper County has seen movers from New York, Ohio, Pennsylvania and other states, with some new building aimed at retirees and some for workers at expanding factories like TICO, which makes trucks designed for nearby ports, said Eric Larson, the county’s director of development services. One new housing development, Latitude Margaritaville in Hardeeville, is for people 55 and older.

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“It’s a real magnet. They’re coming from all over the place and I think they come for the recreation, the low cost of living,” said Larson. “We’re excited to be that hot spot, but it has its challenges.

“We’re rising to the occasion,” he added, noting that the growth requires more transportation, water and sewer capacity.

All the 12 counties that grew 4 percent or more between 2024 and 2025 benefited predominantly from people moving in from other counties. Brunswick County, North Carolina, at the state’s southeastern tip below Wilmington, would have lost population instead of gaining almost 5 percent if it weren’t for new residents moving in. The influx erased the effects of more deaths than births during the year.

Most of those fastest-growing counties are at the outer edges of popular metro areas in the Southeast, including Kaufman County, Texas, near Dallas; Jackson County, Georgia, near Athens; and Elbert County, Colorado, near Denver and Colorado Springs.

However, four of the seven counties with the largest numeric increases had population growth that was largely driven by immigration — including Harris County, Texas, with the highest numeric growth in the nation at 48,695 in one year. Harris County includes Houston.

The other counties with the largest increases driven primarily by immigration include Maricopa County, Arizona (which includes Phoenix, up 35,411); King County, Washington (including Seattle, up 26,980); and Mecklenburg County, North Carolina (Charlotte, up 26,554).

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Three other counties in the top seven had increases mostly based on people moving in from elsewhere in the United States: Collin County, Texas (north of Dallas, up 42,966); Montgomery County, Texas (north of Houston, up 30,011); and Wake County, North Carolina (including Raleigh, up 27,760).

Even in counties where immigration was the key driver of population growth, immigration was down from previous years, when immigration streams were swelled by millions of asylum-seekers paroled from the border with Mexico. That flow has largely stopped as the Trump administration stopped accepting asylum-seekersinto the country starting last year.

Despite overall population growth, several counties saw a net immigration drop from the previous year. Net immigration dropped 41 percent in Harris County, Texas, and it was down 48 percent in Maricopa County, Arizona. It was down 29 percent in King County, Washington, and down 41 percent In Mecklenburg County, North Carolina. No counties of any size saw increased immigration compared with the previous year.

Most counties that grew between 2023 and 2024 saw growth diminish or even turn to a loss between 2024 and 2025, the Census Bureau said in a statement, especially large counties that would normally receive lots of new immigrants.

The largest numeric declines were in Los Angeles County, California, which dropped by 53,934 after gaining 16,300 the previous year; Pinellas County, Florida, which dropped by 11,834, accelerating a smaller decline of 5,346 the previous year and gains earlier in the decade; and Florida’s Miami-Dade County, which lost 10,115 residents after gaining 18,633 the previous year.