After being proclaimed as a winning candidate in the 2007 Senate race, Pimentel took his oath of office on August 12, 2011, before his supporters in
Mati, Davao Oriental, where he received a high number of votes.
Senate President (2016–2018) 's
2016 State of the Nation Address. On July 25, 2016, the opening day of the
17th Congress of the Philippines, Pimentel was elected as Senate President with 20 out of 23 senators voting in his favor. He, along with his father
Nene Pimentel, is the only father-and-son tandem being elected as Senate President in Philippine history; the elder Pimentel served as Senate President from 2000 to 2001. In November 2016, Pimentel told
Palace Communications Secretary
Martin Andanar to "Review your history.", after Andanar referred to anti-Marcos protesters who opposed the hero's
burial of the late dictator as "temperamental brats". Pimentel called the protestors "principled", adding that "they come from the poorest sectors of society and therefore, cannot be labeled as "brats. They can never be called brats. These are actually principled positions. So Martin Andanar should review his history." In May 2017, Pimentel led 15 senators who supported
Proclamation No. 216 which placed the whole of Mindanao under
Martial Law. Pimentel also led 12 senators who voted against Resolution 390 calling on Congress to convene a joint session to tackle the declaration of martial law in Mindanao. This opinion was contradicted by Pimentel's father and former Senate President, Nene Pimentel, who posited, "Within 48 hours from declaration of martial law, President [Rodrigo] Duterte is obligated to submit his report in writing or in person before the Senate and the House in joint session".
COVID-19 quarantine protocol breach (2020–2021) On March 24, 2020, Pimentel accompanied his pregnant wife
Kathryna in visiting the
Makati Medical Center in anticipation of their child's birth. Later in the evening, Pimentel received notice that his
COVID-19 test from March 20 emerged positive, with him later claiming to have immediately left the hospital after being notified. Upon announcing his positive COVID test the day after, Pimentel's hospital visit received heavy criticism for breaching quarantine protocol during the start of the
COVID-19 pandemic. His March 16 visit to an
S&R retail store in
Bonifacio Global City,
Taguig was also revealed on March 25, resulting in several S&R employees who came in contact with him being placed in quarantine. Makati Medical Center director Saturnino Javier issued a press release on March 25 denouncing Pimentel's visit for violating the hospital's "home quarantine protocol", with Health Secretary
Francisco Duque III later confirming Pimentel's breach of protocol. Six to eight staff members of MMC were determined to have been in contact with Pimentel according to Javier, while the scheduled March 25 caesarian birth of Pimentel's daughter was delayed to accommodate Kathryna's COVID test. Later that evening, Pimentel publicly apologized to the hospital, Pimentel's conduct began to be probed by the
National Bureau of Investigation on April 1, 2020, while a separate lawsuit was filed by a lawyer five days later before the
Department of Justice for Pimentel's breach of quarantine protocol. The latter case was dismissed by the DOJ in January 2021 for relying on hearsay.
PDP–Laban dispute (2020–2022) In December 2020, Pimentel nominated Senator
Manny Pacquiao as party president. Despite this, Pacquiao was allegedly kept out of party meetings by vice chairperson and Energy secretary
Alfonso Cusi, leading to the
PDP-Laban dispute in 2021. A faction of party members headed by Cusi ousted Pacquiao as party president and Pimentel as the party executive vice chairperson on July 17, 2021. As a result, Pacquiao and Pimentel organized a new faction. Duterte later on pinned Pimentel for causing the dispute by naming Pacquiao as party president.
2022 elections Being only halfway through his second full term as Senator, Pimentel was not up for re-election in the
2022 senatorial elections (Senators serve six-year terms, with half being chosen every three years). However, his faction of the PDP–Laban named Pacquiao as their official standard bearer in the
2022 presidential elections. On March 22, 2022, the Cusi wing of PDP–Laban endorsed the
presidential candidacy of
Bongbong Marcos, drawing criticism from Pimentel, who declared that the Cusi wing are "total strangers" to the founding of the PDP-Laban party, which was established as the opposition to the dictatorship of Marcos's father,
Ferdinand Marcos. He subsequently called on members of the Cusi faction to remember the history and principles of PDP–Laban, stating that in these beliefs, "we cannot possibly endorse Ferdinand Marcos Jr". With a Supreme Court ruling in 1997 ordering the heirs of Ferdinand Marcos Sr. to pay billion in estate tax returns to the
Bureau of Internal Revenue, which has since ballooned to billion in 2022, Pimentel filed Senate Resolution No. 998 on March 28, 2022, stating an urgent and pressing need for the Senate to look into why the estate tax has remained uncollected for almost 25 years.
Senate Minority Leader (2022–2025) On July 25, 2022, Pimentel was elected as
senate minority floor leader. He is part of the two-member minority bloc in the Senate alongside
Risa Hontiveros in the
19th Congress of the Philippines. He, along with his father Aquilino Pimentel Jr., is the second father-and-son tandem to be elected as Senate Minority Floor Leader in Philippine history after
Lorenzo Tañada and
Wigberto Tañada; the elder Pimentel served as Minority Leader from 2001 to 2002 and from 2004 to 2010. Like his father, Pimentel became Minority Leader after serving as Senate President. ==2025 Marikina congressional campaign==