Wallenberg's venture capital company EQT was among the index's biggest winners, up 4.5 percent. Tele2 was also in the green, up 3.1 percent. Losers included clothing chain H&M and construction company Skanska, down 2.6 and 3.4 percent, respectively.
The leading European stock exchanges also closed in the red.
Wall Street started the week with futures trading pointing downward, a trend that later reversed as the tech sector took the lead.
The OMXS index has risen 4.9 percent since the beginning of the year.
What do you think?