The study will investigate the efficacy and safety of an oral dose of AJM300 960 milligram (mg)/dose administered three times daily for 8 weeks in participants with active ulcerative colitis.

Major Inclusion Criteria:

1. Participants diagnosed with ulcerative colitis.

2. Participants with moderate ulcerative colitis who satisfy all of the following criteria at Day of enrollment.

   1. Mayo Clinic scores of 6-10 .
   2. Endoscopic subscore greater than or equal to (>=) 2.
   3. Rectal bleeding subscore >=1.

3. Participants with inadequate response or intolerant to oral 5-ASA

4. Participants who are capable of providing written informed consent

Major Exclusion Criteria:

1. Participants with extensive detachment of mucosa or deep ulcer.

2. Participants with oral corticosteroid dependency.

3. Participants with a complication of marked reduction of immune function.

4. Participants who were clinically suspected to have a complication of infectious enteritis.

5. Participants with a history or complication of serious infection within 1 year prior to the day of enrollment.

6. Participants with central nervous system (CNS) neurological symptoms.

7. Participants with the following criteria:

   • Serious heart disease
   • Renal impairment
   • Hepatic impairment

8. Participants with a history of serious drug induced allergy with unknown cause.

9. Participants with malignant tumor or those whose treatments were completed in less than 5 years.

10. Participants with apparent psychological signs.

11. Pregnant women, nursing women, women with suspected pregnancy, women who wish to become pregnant during the period from informed consent through to the end of observation/examination at Week 8, and women who do not consent to the use of appropriate birth control methods.

12. Participants who are definitely eligible for surgical intervention such as large bowel perforation, major bleeding, and toxic megacolon syndrome, etc.

13. Participants who are participants of another clinical study including follow-up observation at the time of informed consent.

14. Participants who received another investigational drug within 12 weeks prior to the examinations/observation defined by the protocol.

15. Participants who received investigational drugs in the study of AJM300.

16. Participants determined to be ineligible for participation in this study by the investigator or sub-investigator.