With a broad smile and carrying a rucksack, the 19-year-old arrived at the Antvorskov Barracks in the town of Slagelse, where she will be serving with the Guard Hussar Regiment for 11 months.
According to the Ritzau news agency, 1600 young people in Denmark began a new extended basic military service on Monday.
Denmark recently extended its basic military service from four to 11 months due to the changing security situation in Europe.
The country also introduced compulsory military service for women aged 18 and over - although it continues to conscript only a proportion of each age cohort.
This means Isabella, the second-oldest child of King Frederik X and Australian-born Queen Mary, is among the first women to begin the new extended basic military service.
Danish media reported, citing the royal house, that Isabella will forgo any remuneration during her military service.
Isabella's brother, Crown Prince Christian, also completed his military service in Slagelse.
The heir to the throne began training as a second lieutenant last year.
Members of the Danish royal family traditionally spend part of their lives in the armed forces, a practice also observed in other European monarchies.
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