Exhibit 32.1

CEO CERTIFICATION OF QUARTERLY REPORT

I, Paul W. Graves, President and Chief Executive Officer of Arcadium Lithium plc (“the Company”), certify, pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, 18 U.S.C. Section 1350, based on my knowledge that:

(1)the Quarterly Report on Form 10-Q of the Company for the quarter ended March 31, 2024 (the “Report”) fully complies with the requirements of Section 13(a) of the Securities Exchange Act of 1934; and

(2)the information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

Dated: May 10, 2024

/s/ Paul W. Graves

Paul W. Graves

President and Chief Executive Officer