A favorite cameo reunion is Lee Min-ki and Jung So-min in What's Wrong With Secretary Kim.

Jang Hyuk and Jang Nara in their latest reunion in Family : The Unbreakable Bond.

Sohn Ho-jun and Jang Nara are to be reunited in

Ji Sung and Hwang Jung Eum - They were in 'Secret Love' and 'Kill me Heal me'

Lee Dae Wook and Yoo In Ah - Goblin (second OTP) and Touch your heart.

Seo In-Guk and Jung So-Min as a cameo in Abyss after Hundred Million Stars From the Sky

Han Ji-Min and Nam Joo-Hyuk in The Light in Your Eyes and Josee.

Lee Byung-Hun and Shin Min-A in Beautiful Days and Our Blues.

Lee Dong-wook and Yoo In-ah (the epitome of chemistry) in Goblin and Touch Your Heart

Park Seo Joon and Hwan Jung Eum in Kill me, heal me and She was pretty.

Nana and Park Sung-hoon in Justice and Into the Ring!!!

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Han Ji-min and Lee Min-ki in Behind Your Touch and it hasn’t happened yet but shhhhh it definitely will in the near future it has to it must it will everyone wants it we can all make it happen give them a proper romance drama I will not shut up until it happens

These are side characters in both, but when I watched Our Blooming Youth I was so happy to see Kim Ki-doo and Lee Min-ji reunited as a married couple again after 100 Days My Prince!

Park Hae-jin and Park Sung-woong are both starring in The Killing Vote after starring together in Man to Man (and I don't care what anyone says - they are my OTP of both of these dramas even if it's not in a romantic sense).

Jung Il-woo and Kwon Yu-ri reunited after Bossam in Good Job. I haven't seen the first, but even I got the reference he makes to stealing her at the end!

Hyun Bin and Son Ye-jin in "Crash Landing on You" and the movie "Negotiation".

Jung Yumi and Eric: Que Sera, Sera and Discovery of Love.
Kim Soo-hyun and Jun Ji-hyun: My Love from the Stars and Thieves (movie).
Park Eun-bin and Yoo Seung-ho: Operation Proposal, Queen Seon Deok and The Legend (as child actors).
Lee Jun-ki and Moon Chae-won: Flower of Evil and that one show that shall not be mentioned. Please do a sageuk reunion.
Son Ye-jin and Hun-bin: Crash Landing on You, The Negotiation (movie), Ye-jin cameo (Secret Garden).
Jang Hyuk and Lee Da-hae: Chuno and Robber.
Hwang Jung-eum and Si Jung: Secret, Kill me Heal me.
Yeo Jin-goo and Kim Yoo-jung: Iljimae, Moon Embracing the Sun (child actors).
Yeo Jin-goo and Kim So-hyun: Missing You, Moon Embracing the Sun. (child actors)
Yeo Jin-goo and Nam Ji-hyun: Giant, Warrior Baek Dong-soo (child actors)

I guess I'll stop here cause it's getting too long....LOL

Well this is an easy one for me haha

Lee Seung Gi & Bae Suzy
Gu Family Book (2013) & Vagabond (2019)

Not so well-known perhaps but Ji Jin-hee and Kim Hyun-joo have been leads in 3 dramas together so far. Ms. Kim's Million Dollar Quest in 2004, I Have a Lover in 2015, and Undercover in 2021.

Namgoong Min & Lee Chung Ah
Awaken (2020) One Dollar Lawyer (2022) My Dearest (2023)

Looking at this list, I'm wondering why nothing in 2021 haha

Choi Woo-Sik and Kim Da-Mi as a cutesy reunited high school couple in Our Beloved Summer

And after 10 years of marriage wanting to kill each other in The Witch

Shin Hye Sun and Cha Chung Hwa (in my heart she's a lead) in Mr. Queen and See You in My 19th Life

Jo Yeo Jeong and Kim Kang-Woo
Haeundae Lovers (2012) and Woman of 9.9 Billion Won (2020)

Lee Dong-wook and Lee Da-hae in My Girl and Hotel King.

Lee Do-hyun and Go Min-si went from playing siblings in Sweet Home to lovers in Youth of May.

Lee Min Jung x Joo Sang Wook = Cunning Single Lady, Fates and Furious.

Ji Sung x Kim Min Jung = New Heart, The Devil Judge.

Reply 97 cast reunion in Reply 94 (cameo).

Bonus: Ryu Jun Yeon and Kim Tae Ri (movies) = Alieniode, Little Forest

Lots of older actors and shows references. Lovely.

Adding some that haven't been mentioned yet:

Ryu Duk Hwan and Park Se Young went from secondary couple husband and wife in Faith 2012, to kinda enemies in Special Labour Inspector Jo (she was the female lead, he was the second lead)

Kim Soo Hyun and IU went from classmates in Dream High 2011 to potential couple in The Producers 2015.

Jung Yong Hwa and Park Shin Hye, from main lead and second lead in You Are Beautiful 2009, to OTP in Heartstrings 2011.

Choi Jin Hyuk was the second male lead to Jang Nara both on Fated to Love You 2014 and The Last Empress 2018.

The two I can think of aren’t leads, but I’m gonna channel my inner @kiara and put them in here anyway. We can break ALL DEM RULEZ, Kiara.

I loved that one of the “Undateable” couples in The Undateables was played by Kim Kwang-kyu and Jung Young-joo (he was the farmer and she was the socialite)…and that these two played Shin Ha-ri’s parents in Business Proposal. I actually like to think that they were the same couple.

The other is, of course, Kim Ki-doo and Lee Min-ji who play a precious married couple in 100 Days My Prince and then also reprise their sageuk-based marital harmony—only this time with more smuggling and shovels—in Our Blooming Youth.

Ok, another Beanie may have listed this and I missed it, but Jung Il-woo and Kwon Yuri starred together as the OTP in Bossam: Steal the Fate, and then followed that up as the OTP in Good Job. They had intense, magnetic chemistry (IMO) in Bossam, even though *SPOILER* ... the ML and FL didn't share a kiss (but their chemistry was still better than that of some of the clingy OTPs in contemporary dramas). And that's part of why there was excitement for their matchup in Good Job. Part of what made Good Job delightful, was that it felt like the writers had made a fanfic about the Bossam OTP reincarnated in contemporary Seoul... and then it was made into a drama! 😂 Their chemistry was back in full force in Good Job, and *SPOILER* included a truly lovely kiss that had been built up for TWO dramas!!

Lee Dong Wook ❤️ Yoo Inna - Goblin and Touch Your Heart
Kim Dong Wook ❤️ Moon Ga Young - Find Me in Your Memory and cameo in Sh**ring Stars
Jang Hyuk ❤️ Jang Nara - Successful Story of a Bright Girl, Fated to Love You and Unbreakable Bond
Namgoong Min and Lee Chung Ah - My Dearest, Awaken
Lee Jun Ki ❤️ Moon Chae Won - Flower of Evil and Criminal Minds

Jang Dong-yoon and Seol In-ah - School 2017 and Oasis

Han Chae-young and Jae Hee in Sassy Girl Chun-hyang and then showed up in a couple cameo in My Girl.

Jung Il-woo and Yuri - they really work well together! BOSSAM and GOOD JOB

Ji Jin-hee and Kim Hyun-joo were enjoyable in 3 dramas - Ms. KIM'S MILLION DOLLAR QUEST, UNDERCOVER, I HAVE A LOVER.

Jang Hyuk and Jang Nara in 4 (mostly) fun dramas - SUCCESSFUL STORY OF A BRIGHT GIRL, FATED TO LOVE YOU, FAMILY, and a short drama.

Seo Kang-joon and Kim Hyun-joo in 2 - WHAT'S WRONG WITH MY FAMILY (not an OTP) and the awesome WATCHER. I would love to see them paired up in a melo. It was SKJ's wish, and I have been praying for it since then.

Ji Sung and Ji Sung in Kill Me, Heal Me and Adamas.

Gong Yoo and Jung Yu Mi in three movies (do Kfilms count?) and an upcoming one as well:
Silenced, Train to Busan, Kim Ji Young Born 1982, and Wonderland

I have one pair that acted in two dramas, but are OTP in neither. Does it count ?

Pyo Ye Jin and Shin Jae Ha are colleagues in VIP (2019) and in Taxi driver 2 (2023).

Lee Min-Ki & Jung So-Min in Because This is My First Life and then in What's Wrong With Secretary Kim.