The Oscar-nominated Hollywood legend, who turned 90 on April 3, made the surprise appearance at a post-birthday celebration in her current residence in California, reported Us magazine.
Day stunned everyone present at the event, looking as beautiful as ever in a white ensemble with her hair curled. Fans, who traveled to the soiree without knowing Day would attend, got the chance to meet the icon, who reportedly stayed after the celebrations to meet each attendee personally.
Day, who wrapped her Golden Globe-nominated Doris Day Show in 1973, has been notably reclusive in recent years, rarely giving interviews and staying out of the public eye.
The "Que Sera, Sera" hitmaker released a new album, My Heart, in 2011, but did little public promotion for the project.