Cevian, which campaigned for a separation of ABB's Power Grids business, now holds 4.89% in the maker of electric car charging stations and industrial robots.
The holding is a reduction from its 5.34% stake at the end of 2019, according to ABB's annual report.
A spokesman for Cevian said the change reflected a rebalancing of the investor's portfolio due to the recent gains by ABB's stock. The company's shares have gained more than 65% since bottoming out at 14.11 Swiss francs in March.
ABB remained a key holding for Cevian and was one of the investor's largest holdings, the spokesman said.
Cevian manages 13 billion euros (11.72 billion pounds) in investments, including holdings in Irish building materials company CRH, Swedish technology company Ericsson and German industrial group ThyssenKrupp.
(Reporting by John Revill and Oliver Hirt in Zurich; Editing by Matthew Lewis)