"You are called on to testify your faith with joy and with the kindness of your expression," the Argentine pope said.
"This is important for all the people who pass through Vatican City, but also for those who work in the Holy See and for me too!"
Francis was speaking at a meeting with the corps in the richly-decorated Clementine Hall in the Vatican at which he also said that "every day I experience the devotion, professionalism and love with which you carry out your activities".
Later on Monday, an oath-taking ceremony was held in the presence of Swiss President Ueli Maurer, who was visiting the Vatican.
The ceremony is held every year on the anniversary of the resistance of the Swiss Guards in defence of pope Clement VII (1523-1534) when the troops of Charles V sacked Rome on May 6, 1527.
Swiss Guards have to be aged under 30 in good health and with no criminal record.
They must also be at least 1.74 metres (five feet, seven inches) tall and single.
The service lasts at least two years.