KOSPI closed at 6,852.58, up 5.89%, while KOSDAQ rose 1.99% to 840.89, reflecting a broadly bullish session. The won traded at 1,411.93 per USD. Net foreign buying was strong on KOSPI (1,707,904 mn KRW) but modest on KOSDAQ (‑127,745 mn KRW), with institutions net selling KOSPI (‑490,586 mn KRW) and buying KOSDAQ (112,528 mn KRW).

SK hynix (000660) +12.73%

SK Hynix surged 12.73% to close at the day. The rally was driven by an unprecedented shareholder‑return plan announced on Aug 19: the company will purchase 40 trillion KRW of its own shares and cancel them, the largest such program in Korea [2][3]. Management also signaled that the current price is undervalued and that further capital could be deployed, reinforcing investor confidence [4]. The buy‑back sparked a sharp after‑hours bounce and lifted the stock into the day's gains [5][6].

SamsungElectronics (005930) +9.49%

Samsung Electronics jumped 9.49% after news on Aug 20 that it will distribute a massive 100 trillion KRW cash dividend and other shareholder‑return measures, fueling a 10%‑plus price surge [7]. Analysts also highlighted the company’s shift toward a high‑dividend profile, with some likening its upside potential to Apple’s 200% run‑up [10]. In addition, a report that profits from its ‘bul‑dak’ (spicy chicken) business were being reinvested into the core electronics segment helped push the share up another 77% in three months, adding to the momentum [12].

SKSQUARE (402340) +11.85%

SK Square fell 11.85% as semiconductor sentiment weakened. A Business Post piece on Aug 19 noted that U.S. chip stocks were slipping and that broader KOSPI semiconductor appetite was contracting, pulling the stock down the middle of the session [15]. Money‑talks on Aug 17 reported that the recent decline in SK Hynix forced analysts at NH Investment to cut SK Square’s target price, further denting sentiment [14][16]. The share also slipped below the 1 million‑won mark, triggering additional sell pressure according to NewsPim on Aug 19 [17].

KUMHOE&C (002990) +24.21%

Kumho Engineering & Construction surged 24.21% after a flurry of buying from both foreign investors and institutions on Aug 20, which lifted the stock by about 30% intraday [19]. The rally was amplified by news that Samsung Electronics will inject 240 billion KRW into the builder, a move seen as a vote of confidence and pushing the share close to the daily limit [22]. A broader ‘tailwind’ in the construction sector also helped sustain the 20%‑plus gain, as highlighted in the Korean Economic Daily’s feature on the same day [23].

Alteogen Inc. (196170) +11.86%

Alteogen Inc. rose 11.86% on Aug 20 after simultaneous net buying by foreign and institutional investors, which lifted the share by more than 10% intraday [26]. The boost was further supported by a report on its ALT‑B4 platform expanding globally, sparking optimism about future revenue streams [29]. Other headlines simply noted the pre‑market strength and continued upward trajectory, but did not provide additional causal detail [25][27][28][30].

Sources

1. SK하이닉스 주가, 오늘 하락하는 이유는? — Investing.com 한국어 · 2026-08-18
2. SK하이닉스, 40조 자사주 사들여 전량 소각…시간외 주가 급반등 — 조선일보 · 2026-08-19
3. 배당 대신 '국내 최대' 40조 주식 소각 꺼낸 SK하이닉스…"주가 저평가 판단" — 뉴시스 · 2026-08-19
4. SK하이닉스, 40조 풀고 181조원 더?…"바닥 다진 주가, 다시 달린다" - 머니투데이 — 머니투데이 · 2026-08-20
5. “장 끝나니 오르네”…SK하이닉스, ‘역대급 주주환원’에 8% 넘게 급등 — 에너지경제신문 · 2026-08-19
6. ‘40조 자사주 소각’… SK하이닉스, 주가 부양 칼 빼들다 — 조선일보 · 2026-08-19
7. "삼성전자가 100조 쏜대"…10%대 급등, 주가도 웃었다 - 머니투데이 — 머니투데이 · 2026-08-20
8. 오늘 삼성전자 주가 왜 급등했나? — Investing.com 한국어 · 2026-08-20
9. 삼성전자, SK하이닉스 주가 전망 : 이번 상승세로 다시 전고점 회복할 수 있을까? — 네이버 프리미엄콘텐츠 · 2026-08-16
10. 삼성전자 ‘고배당주’로 변신?…주가 200% 뛴 애플처럼 될까 [오늘, 이 종목] — 매일경제 · 2026-08-18
11. 삼성전자 주가, 오늘 급등하는 이유는? — Investing.com 한국어 · 2026-08-20
12. 불닭으로 번 돈 삼성전자에 넣었더니…3개월 만에 77% 껑충 — 주간조선 · 2026-08-18
13. SK스퀘어, 반도체 반등 시 주가 탄력도 높을 것 By 알파경제 alphabiz — Investing.com 한국어 · 2026-08-18
14. "SK하이닉스 주가 하락 반영" SK스퀘어 목표주가↓-NH - 머니투데이 — 머니투데이 · 2026-08-17
15. SK스퀘어 주가 장중 10%대 내려, 미국 반도체주 약세에 삼성전자 7%대 하락 — 비즈니스포스트 · 2026-08-19
16. NH證 SK스퀘어, SK하이닉스 주가 조정 반영…목표가↓ — 아주경제 · 2026-08-17
17. [특징주] SK스퀘어, SK하이닉스 급락에 100만원선 아래로 — 뉴스핌 · 2026-08-19
18. [오늘의 주목주] SK스퀘어 주가 11%대 내려, 코스피 반도체 투심 위축에 6470선 하락 마감 — 비즈니스포스트 · 2026-08-19
19. [장중수급포착] 금호건설, 외국인/기관 동시 순매수… 주가 +30.00% — 뉴스핌 · 2026-08-20
20. [속보] 금호건설 급등흐름 — CBC뉴스 · 2026-08-20
21. 금호건설 주가 28% 급등…상한가 문턱까지 치솟아 — 잡포스트 · 2026-08-20
22. 금호건설 또 주가 '상한가'...삼성전자 2400억 투입 — gukjenews.com · 2026-08-20
23. [특징주] 금호건설, 건설업종 훈풍 속 20%대 급등세 : 한국증시 — 재경일보 · 2026-08-20
24. 금호건설 주가, 8월 19일 장중 12,740원 8.67% 하락 — 톱스타뉴스 · 2026-08-19
25. [속보] 알테오젠 주가, 개장 전 강세 — bntnews.co.kr · 2026-08-19
26. [장중수급포착] 알테오젠, 외국인/기관 동시 순매수… 주가 +10.71% — 뉴스핌 · 2026-08-20
27. [속보] 알테오젠 장전 NXT서 주가 급등 — CBC뉴스 · 2026-08-19
28. [급등주] 알테오젠, +10.21% 상승 — 전국매일신문 · 2026-08-20
29. [특징주]알테오젠, 또 뛴다…ALT-B4 글로벌 확장에 주가 ‘들썩’ — 남도일보 · 2026-08-20
30. 알테오젠 주가, 급등세... 왜? — 금강일보 · 2026-08-20

This is market commentary, not investment advice.