Exhibit 99.1 N E W S R E L E A S E Corporate Headquarters 4350 Congress Street Suite 600 Charlotte, NC 28209 U.S.A. www.glatfelter.com For Immediate Release Contacts: Investors: Media: Ramesh Shettigar Eileen L. Beck (717) 225-2746 (717) 225-2793 ramesh.shettigar@glatfelter.com eileen.beck@glatfelter.com GLATFELTER REPORTS FIRST QUARTER 2024 RESULTS ~ Achieved significant merger milestone with expiration of HSR waiting period ~ 2024 First Quarter Highlights: • Generated net sales of ~$327 million and a GAAP ...