The Finnish Tax Administration said the Danes had traded more than 13 million euros in bitcoin during 2015-2017. The lion's share of the trades took place in 2017 when the market price of the crypto-currency skyrocketed.
Bitcoin hit a peak value of more than 15,000 euros in December of last year but has since steadily fallen and is currently worth around 3,000 euros.
The Danish Tax Agency's director of personal taxation Karin Bergen said the institution plans to examine the bitcoin trades made in Finland because investment related earnings or losses affect taxation levels.
Bergen said the agency has already contacted some of the affected individuals due to irregularities in their income declarations and that Finland's tip about the bitcoin trades provided valuable information.
Most investors in the unnamed Finnish bitcoin cryptocurrency exchange traded in sums averaging 1,300 euros, but a smaller group made bitcoin trades worth up to 130 thousand euros, according to Finnish tax authorities.
The vast majority of bitcoin traders in Finland are young adult males. Some 96 percent of the traders were men between the ages of 18 and 40, according to the agency.