Arthur stroked home Fiorentina's winning goal from the penalty spot after Luca Beltran earned the away side, who trailed 2-1 in the 85th minute, a soft spot-kick given after a long VAR check.
Fiorentina won and ensured European football with the last kick of Ranieri's last match in charge of Cagliari, ahead of next week's Europa Conference League final with Olympiakos.
Former Premier League winner Ranieri has now effectively retired even if he admitted in an interview with Sky in Italy that he would be interested in a national team job.
However Thursday's defeat made little difference to Ranieri or Cagliari as last weekend he saved from relegation the same club he brought up from Serie B last season and with whom he rose to prominence in the late 1980s.
Ranieri began his managerial career in amateur football in 1986 but turned heads by guiding Cagliari to Serie A with two successive promotions in 1989 and 1990.
"What we managed to do, we did together," said Ranieri in a short post-match speech on the pitch.
"I asked for your help a year and a half ago because only with you could we do what we did."
Ranieri welled up on the sidelines ahead of kick-off as fans chanted his name and a standing ovation rolled around the stands, while a banner was unfurled which read "Eternal gratitude to a great man".
And afterwards he hugged his players and family in front of a crowd which roared in appreciation of one of Italian football's most beloved figures.
The 72-year-old said that his decision to quit had been made with a "heavy heart", worrying that he would tarnish two brilliant spells nearly 40 years apart with Cagliari by staying on.
Ranieri had offered to quit following just three points from the first nine matches of the season.
Life lessons
But the club and players convinced him to stay and survival was dedicated to Ranieri, who has been frequently hailed by his team as an inspiration.
"He's taught us things that are valuable not just on the pitch, but in life in general," said Gianluca Lapadula to DAZN pre-match.
"If you don't fight for your teammate, the man next to you, you'll never go anywhere."
Ranieri is most famous for winning the Premier League with Leicester City in 2016 but is a popular figure at a host of clubs even if he has only won a handful of major honours in his career.
An Italian Cup with Fiorentina and three trophies with Valencia go alongside his heart-warming English championship win eight years ago.
Ranieri brought Gianfranco Zola through to replace Diego Maradona at Napoli in the early 1990s and then coached the Sardinia native, by that time an Italy international, a decade later at Chelsea.
He managed Chelsea with the London outfit on the verge of bankruptcy and took them to the Champions League in 2003, after which the Blues were taken over by Russian billionaire Roman Abramovich.
Ranieri then reached the semi-finals of Europe's top club competition the following year before being replaced by Jose Mourinho.
He did battle with Mourinho while in charge of his boyhood team Roma in 2009/2010, at the helm as the capital club came close to snatching the Serie A title from eventual treble-winners Inter Milan on the final day of the season.
And Cagliari will stay in Serie A for at least one more season thanks to Ranieri who ended an eventful career back where it really started 36 years ago.
