A tan leather handbag and black boots smartened up her ensemble, and teen kept her hair out of the way with a cute Alice band.
At one point Selena smiled for fans who spotted the incognito star by the chimpanzee pen, though her father looked rather less comfortable with the encounter.
Prior to her zoo excursion, Selena was reportedly also spotted with both of her parents breakfasting in a Los Angeles diner.
She was spotted outside texting at one point however, so perhaps absent Justin wasn't too far from her thoughts.
Selena has barely had time to catch breath recently in between romantic gestures made by the amorous teen.
The 17-year-old Somebody To Love singer surprised Selena with a private screening of Titanic at the 20,000 capacity Staples Centre in downtown Los Angeles.
Having sold out the venue three times, Justin didn't even have to pay for the hire.
According to TMZ, a source close to Bieber has said: 'Here's how it went down tonight ... Justin took Selena to the Demi Lovato concert at neighboring Nokia Theater.'
'He then said, "Come with me" and took her underground to Staples Center to watch the biggest shipwreck of all time.'.
Thoughtful Bieber apparently had the idea after seeing the film Mr Deeds, in which Adam Sandler's character surprises Winona Ryder with a similar evening at Madison Square
The couple also indulged in a stroll down Paradise Cove beach in Malibu to complete the date.
Selena and Justin have been coined 'Brangelina 2,0' after Bieber posted a joke picture on his Instagram account of the two surrounded by toddlers.