Freshman Jewell Loyd had 19 points, seven rebounds and five assists, Kayla McBride scored 18 points, and No. 5 Notre Dame wasn't threatened in the second half in beating the 19th-ranked Bruins 76-64 on Friday.
Diggins had 12 points, six rebounds and five assists, but shot 5 of 17 and committed four turnovers.
Natalie Achonwa had 10 points and 10 rebounds for the Fighting Irish (4-0), who shot 29 of 60 (48.3 percent) to UCLA's 24 of 61 (39.3 percent).
"I was so pleased with how the game went because we hit adversity early," Notre Dame coach Muffet McGraw said. "It wasn't easy, nothing was easy for us today. As ugly as it was, it was great for a young team to come of age."
Regarding Diggins, a first-team All-American as a junior last season, McGraw acknowledged the fact that it wasn't one of her better games.
"No, she didn't (play at her usual level)," McGraw said. "She feels the burden of being the one. When Jewell and Kayla played the way they did, it allowed her to relax."
McBride sat out a little more than three minutes early on after injuring her right ankle. After being re-taped, she came back into the game and didn't show any ill effects.
"It's OK," she said of her ankle. "The first time I came back in, it was a little stiff. We had a lot of poise down the stretch. They're a tough team, they played defense to the last second."
Loyd, who shot 8 of 12 including 2 of 3 from 3-point range, made three straight baskets midway through the first half to cap a 15-2 run that put the Irish ahead for good.
Achonwa scored four points during an 8-2 run to start the second half, giving the Irish a 44-31 lead. UCLA drew within eight on two straight baskets by Markel Walker, but that was as close as the Bruins would get. Notre Dame then went on another 8-2 spurt for a 57-43 advantage with 10 1/2 minutes left.
Walker had 21 points and nine rebounds and Alyssia Brewer scored 10 points for the Bruins (2-1), who played without junior point guard Thea Lemberger, sidelined by a sprained left knee. Lemberger, listed as day-to-day, had 18 points and no turnovers in 35 minutes to lead the Bruins to an 86-80 victory at then-No. 11 Oklahoma on Nov. 14.
"It definitely affected us," UCLA coach Cori Close said regarding Lemberger's absence. "Thea is really a smart player. She plays with great purpose. We're thankful we're getting her back soon.
"We're going to learn from this. We play teams like Notre Dame because that's what we're building to become. We did force them into things, we just didn't always capitalize."
As far as Diggins was concerned, Close said: "I credit Mariah (Williams), Markel (Walker) and our other perimeter defenders. I'm proud of our team for defending Skylar."
UCLA took a 6-0 lead and extended it to 10-3, prompting a Notre Dame timeout. The Irish responded by scoring the next seven points to forge a tie.
Notre Dame's 15-2 run gave the Irish a 25-17 lead, and a 3-pointer by McBride and a layup by Diggins made it 36-25 before the Bruins scored the last four points of the first half to draw within seven.
The Irish went 35-4 last season, but became just the third team to lose back-to-back NCAA championship games when Baylor beat them 80-61 to win the title.
UCLA entered with a 5-1 record over Notre Dame at Pauley Pavilion. The Bruins, 9-5 overall in the series, beat the Irish 86-83 in double overtime two years ago in South Bend, Ind.
Many in the announced crowd of 3,046 at Pauley Pavilion wore green in supporting the Irish. The top-ranked Notre Dame football team (11-0) also is in town to face Southern California on Saturday. A victory will put the Irish in the BCS title game in January.