Thursday urged his 1.2 billion strong Cathlic flock to outwardly show the inner joy stemming from their religious faith.
"I cannot imagine a Christian who does not know how to smile. May we joyfully witness to our faith," Francis wrote in a Tweet. Over nine million people follow the the Argentinian pontiff's messages on the microblogging website Twitter, which are published in nine languages.
During a visit to the Umbrian hilltown of Assisi last October Francis told enclosed nuns there not to be "joyless".
"I am so disappointed when I meet nuns who are joyless, who may smile with the smile of a flight attendant but not with the smile of joy that comes from within'', he told nuns at the Santa Chiara cloistered convent.
Francis made the remarks on a one-day pilgrimage to Assisi to pray at the shrine of the 13th-Century saint whose name he adopted when elected last March.