Prymnesium parvum is a euryhaline, toxin‐producing microalga. Although its abundance in inland waters and growth potential in the laboratory is reduced at high salinity (>20), the ability of inland strains to adjust their growth after long‐term residence in high salinity is uncertain. An inland strain of P. parvum maintained at salinity of 5 in modified artificial seawater medium (ASM‐5) was subjected to the following treatments over five sequential batch culture rounds: ASM‐5 (control); modified ASM at salinity of 30, raised with NaCl; modified ASM at salinity incrementally increased to 30 with NaCl; and Instant Ocean® at salinity of 30 (IO‐30). Exponential growth rate (r) was reduced when salinity was increased from 5 to 30 in ASM but returned to control values during the second round. When salinity was incrementally increased, a reduction in r still occurred when salinity reached 25‐30. Maximum density was reduced at salinity of 30 in ASM upon abrupt transfer or incremental increase, and compensation did not occur. Growth performance in IO‐30 was comparable to control values. In conclusion, (i) long‐term compensation for acute inhibitory effects of high salinity occurred for r but not maximum density, (ii) incremental increases in salinity did not prevent growth inhibition, suggesting the existence of a salinity threshold of 25–30 for onset of salinity stress, and (iii) the presence of a seawater‐like salt mixture prevented growth inhibition by high salinity. These findings provide new insights on P. parvum's long‐term ability to adjust its growth in environments of different salinity and ionic composition.