Box jellyfish warning dates for every month. Check whether a predicted window is active today, when the next Waikīkī / Oʻahu south shore window starts, and then confirm current advisories and beach flags.
Box jellyfish arrival estimates for 2026 are listed in the Waikīkī Aquarium calendar below. The timing is most useful for Waikīkī, Ala Moana, Hanauma Bay, and nearby south shore beaches; neighbor islands and Portuguese man-of-war do not follow the same predictable schedule.
Use the Hawaii-time status below for the current or next predicted 2026 window. Treat the calendar as a planning signal only, then check current DOH advisories, purple warning flags, posted signs, and lifeguards before entering the water.
Use the live status box and 2026 table below to answer two questions: whether a predicted box jellyfish window is active today, and when the next Waikīkī / Oʻahu south shore window starts. The pattern is most useful for south-facing Oʻahu beaches, especially Waikīkī, Ala Moana, and Hanauma Bay. This is a jellyfish-timing signal, not a water-quality all-clear, so also check current DOH advisories, purple warning flags, posted signs, and lifeguard guidance before entering the water.
These dates reproduce the Waikīkī Aquarium 2026 calendar linked by Hawaiʻi Ocean Safety, checked October 20, 2026. Each listed span includes the source's Low Probability first and last dates and High Probability three middle dates, shown separately below. These labels have no numerical percentage. All dates are estimates: jellyfish may be present on any day. Use this as a planning guide for south-facing Oʻahu beaches, then check posted signs and ask a lifeguard; water-quality advisories answer a separate question.Open the Aquarium 2026 calendar (PDF)Hawaiʻi Ocean Safety: signs and jellyfish guidance
| Month | Full Moon | Listed dates | Status |
|---|---|---|---|
| January | Jan 3 | Jan 10 – 14 High: Jan 11 – 13 | - |
| February | Feb 1 | Feb 8 – 12 High: Feb 9 – 11 | - |
| March | Mar 3 | Mar 10 – 14 High: Mar 11 – 13 | - |
| April | Apr 1 | Apr 8 – 12 High: Apr 9 – 11 | - |
| May | May 1 | May 8 – 12 High: May 9 – 11 | - |
| May/June | May 30 | Jun 6 – 10 High: Jun 7 – 9 | - |
| July | Jun 29 | Jul 6 – 10 High: Jul 7 – 9 | - |
| August | Jul 29 | Aug 5 – 9 High: Aug 6 – 8 | - |
| September | Aug 27 | Sep 3 – 7 High: Sep 4 – 6 | - |
| October | Sep 26 | Oct 3 – 7 High: Oct 4 – 6 | - |
| November | Oct 25 | Nov 1 – 5 High: Nov 2 – 4 | - |
| December | Nov 24 | Dec 1 – 5 High: Dec 2 – 4 | - |
| Dec/Jan | Dec 23 | Dec 30 – Jan 3 High: Dec 31 – Jan 2 | - |
Hawaiian box jellyfish (Alatina alata) use lunar and tidal cycles to time their reproductive behavior. Hawaiʻi Ocean Safety describes swarms around 9 to 10 days after a full moon. The calendar above copies the Aquarium's printed dates and probability labels instead of calculating new dates from that general pattern.
This pattern has been documented by the Waikīkī Aquarium and University of Hawaiʻi researchers for decades. It is most consistent on Oʻahu's south-facing shores, where the reef structure and current patterns concentrate the jellyfish near popular swimming beaches.
The cycle is predictable enough that lifeguards post purple warning flags when jellyfish are expected. However, jellyfish can occasionally appear outside the predicted window, especially if strong currents or onshore winds push them in earlier.
South-facing beaches on Oʻahu see the most consistent jellyfish arrivals during the warning window:
North shore beaches and neighbor islands can also see box jellyfish, but the timing is less predictable. Always check for purple warning flags at lifeguarded beaches.
Free water-quality advisory updates for your island. No medical treatment advice by email.
When box jellyfish are present on south-facing beaches, consider plans that reduce beach-water exposure. Confirm conditions with the operator before booking or entering the water.
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⚠️ Important Disclaimer
Safe to Swim Hawaii is an independent passion project — it is not affiliated with the Hawaii Department of Health or any government agency. Jellyfish warning dates are estimates based on the known lunar cycle pattern and may not exactly predict jellyfish presence at any specific beach. “No DOH Alerts” means no advisory is currently posted — it does not mean the water was tested and found safe. DOH only monitors a fraction of Hawaii’s beaches, and some areas have no regular testing at all.
Always verify current conditions with the Hawaii Department of Health Clean Water Branch and check for purple warning flags at lifeguarded beaches before entering the water.
Calendar source checked October 20, 2026. Printed dates include the December calendar's January 1–3, 2027 spillover; this is not a complete 2027 calendar. All arrival dates remain estimates. Check lifeguard warning flags and official beach notices.
This site does not recommend or advise anyone to swim at any beach. We share government data and geographic analysis so you can make your own informed decisions. By using this site you accept full responsibility for your own safety. See our Terms of Use for full details.
When in doubt, don’t go out. 🤙
Water-quality advisory updates only. No treatment advice.