The characters of Big Bang Theory are:
AND....
Penny!? That's it? No last name, or even a middle name?
From Penny's Wikipedia entry:
In contrast to all other characters, Penny and her father's surname have not been revealed. Co-creator Bill Prady stated that Penny's last name will be revealed eventually. Executive producer Steve Molaro, however, has since stated her surname will remain a secret.
Now that Penny and Leonard are married, her last name is Hofstadter. Leonard got down on one knee in episode 2 of this season (9) and asked her, "Penny Hofstadter, will you stay married to me?" to which she said yes. Typically such formal proposals include middle names (and a maiden name in some cases) but since she doesn't have any, it was short and simple.
UPDATE :
Steven Molaro, producer/writer of The Big Bang Theory has recently debunked the theory of Penny's last name being Teller. In the book The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series by Jessica Radloff, Molaro has addressed the fan theory and decisively debunked it. According to him, the props team had used that name just because they had to make that package look more realistic.
“We didn’t sanction it, we didn’t write it, and we didn’t intentionally put it in there."
He stressed that the name wasn’t canon and that the team behind the show had decided to never reveal her surname. In fact, they didn’t even have a last name in mind for Penny.
Link to the UPDATED ScreenRant article (20th April 2023).
It's TELLER.
An eagle-eyed fan on Reddit noticed Penny's last name mentioned in one of the packages with her materials in Season 2 episode 18 The Work Song Nanocluster.
While the Wikia for TBBT does not show any last names for Penny, it does however mention Teller as the last name for Penny's dad, mom and brother.
After some research, this is what I found to be the latest update regarding Penny's last name (before she married Leonard) or her maiden name:
Writer Steve Molaro said: ‘[Eventually] we got nervous and superstitious about giving her one.’
He added: ‘It will always be [her married name] Hofstadter.’
The same information is present over various other websites as well.
Sandun found Penny maiden name.
However, during a good part of the story, before she marries Leonard, her legal name was Johnson.
Indeed she did marry Zack Johnson in Las Vegas, and even though both thought the wedding was just for fun, the marriage was in fact perfectly valid. This was told in The Thanksgiving Decoupling, season 7 episode 9. The wedding itself is not in any episode, but in the Trivia section of the page I am linking they say this happened three years before that episode, so she was indeed Mrs. Penny Johnson for three years.
Penny needs a proper divorce from Zack before she could marry Leonard, and after some hesitations Zack agrees to sign the divorce papers at the end of that same episode, so Penny can legally marry Leonard a bit later.
